Unforeseen Consequences is a moderately short stage that should be rather easy for most players. It takes place after the incident in the Anomalous Materials labs and the Black Mesa sewer system, where you must find an escape route.
Escaping the Labs
Out of the Test Chamber
You'll begin in the test chamber, which is destroyed after the experiment. Start off by going through the large brown doors to the right, where you will see one of your fellow scientists dead. Walk up to the retinal scanner to the right of the doorway and activate it by pressing your use key. It will start to sputter, but the door will open anyway.
In the next room you'll see a scientist attempting CPR on a fallen security guard. Eventually the security guard will come back to life, and you can make him follow you. However, it is useless to do so, since the security guard will not get on the elevator (same with the scientist). Now take a right from the test chamber entrance. Be careful though, because a machine will fall on a dead scientist, crushing him. Anyway, take a left at the hallway intersection to get to the small elevator.
Getting back to the Elevator
This elevator will lead back to the room where the equipment was destroyed. One of the scientists is hurt, and you'll need one of them to unlock a door. Have the scientist on the right follow you (by pressing your use key) to the Control Room where he will unlock the door. In the original Half-Life, he will stay with the hurt scientist, but in the Source version, he can keep following you. It's useless to have him follow you because he will most likely be killed in the upcoming room and cannot get through a doorway.
In the Control Room, walk past the fallen scientist's body to trigger a large lightning bolt to come from the test chamber. This lightning bolt will cause a lot of damage, so you must avoid it. Do so by hiding behind the small black control panel in the corner, and wait until the lightning destroys the doorway. Time your movements and quickly run past the door. A loadzone will trigger once you do.
Follow the hallway passage until you get to the water pump room. A headcrab will spawn and fall near the first fallen pipe. Run as fast as you can to the next doorway to avoid being bitten by the headcrab. The floor will be slippery, so be careful.
The next area is the laser storage room, and you guessed it, the lasers escaped. Immediately crouch down and walk toward the dead security guard. The lasers should stay above you most of time, so you should have no problem not getting hit. When you reach the end of the hallway, you'll see what happens when a security guard and a laser meet. Anyway, grab the crowbar that's next to the doorway and smash the bottom right glass pane. Crouch under the glass pane to get to the next room.
Now walk up to the elevator doorway and smash one of the bottom glass panes again. When you step through, a group of scientists fall to their doom (you can also press the elevator switch to cause this event). Now climb down the red ladder to the right to go to the bottom. In the corner is a battery, so pick it up. Climb back up the ladder to get to the platform. Now climb up the ladder labeled 'Maintenance Access' to get back to the lab's top floor.
Finding an Escape Route
When you reach the top floor, you'll immediately see a security guard engaging a zombie. Help the security guard by stabbing the zombie with your crowbar. Another zombie will come from the right passage, but the security guard should be able to handle this one with no problem.
Now you'll get your first taste of having a security guard following you. Have him come with you and step through the next doorway. You can kill him with your crowbar to take his Glock pistol, but having him kill the enemies is much safer and easier.
When you step through the next doorway, a loadzone will trigger. Keep following the blue panel and you'll encounter two zombies in the hallway. Have the security guard kill both zombies (they shouldn't reach the security guard in time. If they do, kill them immediately).
Now keep following the blue panel to get to the corridor. Before we go back to the main entrance, let's go back to the locker room. You can grab two clips for the Glock pistol in the open locker (with the American flag). Go to the locker labeled 'Freeman' to pick up two batteries, and use the health station in the bathroom if you need to. Now go back to the corridor and take a left, where a loadzone will trigger. Follow the passage to get back to the main entrance, which is now destroyed.
The security guard should kill the headcrab that's near the small vent. Have him wait at the main entrance (by pressing the use key again) and walk up to the small control panel next to the large brown doors. Smash the control panel with your crowbar and go on through. Wait for the next door to open (you'll have to crouch down to go through both doors) and walk up to the scientist. Step backwards immediately, as the bridge will collapse. Grab the clip next to the fallen security guard and go back to the main entrance.
The security guard that you left behind serves no purpose now, so you can either kill him for extra ammo or just leave him there. Now crouch under the small vent to go into the next room. Use the nearby health station if you have to. There's another vent in the top right corner which we need to get to. Jump on the first fallen machine that's blocking the doorway, and then jump on the one in front of it to get to the vent. Crouch under the vent and go down into the next area.
Avoid the fallen vent in the corner, as a headcrab will jump out of it once you get close. Near the vent is the famous headcrab vs. scientist scene, and the room across from here has a rather frightning display. Now go through the passageway to see a dying security guard (there's no way to save him). Grab his Glock pistol nearby (you may already have one if you killed the first security guard). Now take out your crowbar and crouch under the left glass pane to proceed to the next area.
The Sewer System
Press the switch next to the door to open it, which will cause it to slide back and forth, so time your movements to go through it. Now make sure you have your crowbar out. Take a right at the door and a houndeye will spawn. Start stabbing it with the crowbar as fast as you can. Now take a right from the intersection, and another will spawn. Do the same. There's a scientist nearby, but he can't go far, so there's no use in having him follow you. Take the left passage from the intersection, where two houndeyes will come. Stab the one closest to you, and then immediately start walking backward to avoid the other's sonic attack. When the second one has finished it's sonic attack, run up to it and start stabbing. There's a health station in the room with the red light, so use it if you have to.
Now go up to the door with the yellow hazard sign. Open the door and a headcrab will jump out of nowhere. Kill it immediately, and then follow the narrow passage. Take your pistol out before you go up the ladder. Climb up the ladder so you can see the large explosive tanks, and then shoot them quickly. This will kill the zombie and the two headcrabs that are nearby. To the right of the ladder is a passage that leads to a scientist. Have him follow you to the narrow walkway (which leads to a storage room) on the left, where a zombie is having it's dinner. Kill the zombie and then have the scientist open the door for you. Inside is three clips of pistol ammo, fifteen grenades, and a health station. Now jump down from the walkway (try crouching before you jump down to avoid being damaged) and go down the passageway labeled '19.'
Take your grenades out and start walking backward from the small brown door. The door will start to be ripped apart by a Vortigaunt, and as soon as it's destroyed, throw a grenade in there to kill it. Pick up the two clips of ammo that are in this room, and then kill the headcrab that's hiding behind the barrels. There's also a health station behind the dumpster (not to mention a scientist inside of the dumpster) if you need it.
Go near the small opening and a zombie will emerge from behind the gate. Either throw a grenade in the room or start shooting the zombie to kill it. In the original Half-Life, a headcrab is directly under the opening (you can kill it easily by just throwing a grenade down). However, in the Source version, the headcrab is not there, so you don't have to worry about it. Now drop down into the opening and follow the right passageway until you see a valve handle. Start turning the valve handle (press the use key) and water will start to rise. Swim up to the opening where the zombie came from to get to the next area. Follow the passageway to get to a large elevator (a loadzone will trigger).
Before you pull the lever, grab the two clips of pistol ammo near the dead security guard. I would definitely save before you pull the lever. When you do, the elevator will immediately start to descend...but so do a bunch of headcrabs. Quickly go onto the elevator (you will sustain some damage if you jump over the railing, so just go around). There's two ways to beat this. You can stay on the elevator and battle about twenty headcrabs, or you can just slide down a ramp to the left (or right) and not battle any of the headcrabs. If you do decide on the ramp, I recommend taking the left one because you can have access to another health station. If you fall into the water, you will instantly be killed.
If you took the left ramp, press your backwards key (default: down arrow or S key) once in awhile to slowly stop your fall as you go down. When you get close to a raised platform on the left, do a diagonal jump to reach it, and you can use the nearby health station. To get off the platform, just try to land on one of the barrels when you jump down or just press crouch when you're about to land.
If you took the right ramp, just slide all the way down until you get about a quarter of the way, where you'll then have to start pressing your backwards key to slowly go down (so you don't take any major damage). There's a health station up ahead which you can use. To get to the other side, just jump on the raised gray beams (you can also do the same from the other side if you need both health stations).
Most of the headcrabs will be stupid enough to jump from the elevator, which will cause their immediate death. If you stayed on the elevator, just try to dodge the headcrabs as best you can, and shoot or stab any that get close. You can also try throwing grenades to eliminate larger groups of them. Whichever path you took, every one of them leads to the same ground floor. When you walk past the largest wooden crate, a houndeye will spawn. Just smash the crate with your crowbar and then quickly kill the houndeye when it drops down. Smash two of the larger wooden crates nearby for two clips of pistol ammo.
Follow the nearby tunnel that leads to a small bridge. Don't go too far, because as soon as you start walking across, a bullsquid will spawn and cause the bridge to be destroyed. You'll have to jump to some nearby white pipes, which lead to small vent shaft you can use to get to the other side. From the bridge, jump to the pipes on the right and start climbing the other pipes until you get to the corner. From there, start going across the pipes that lead to the vent shaft. Be careful not to fall when you go across, as the pipes are rather thin. Smash the vent case and crouch down. Keep moving until you come to the second vent case, and then destroy it to get to the other side of the tunnel (the first will make you fall from a large height, which takes a lot of your health away).
Follow the passage and destroy all of the small wooden boxes to collect three batteries. Down below you will see a bullsquid fighting some headcrabs. Try not to be spotted by the bullsquid, as the bullsquid will actually attack the headcrabs for you. If you get it's attention, it will then focus on you. After the bullsquid is done killing all of the headcrabs (or at least some), start throwing grenades at it. It should take about 2-3 grenades to put it down for good. If you're out of grenades, then just use your pistol, dodging the bullsquid's acid spit as best you can. Kill any remaining headcrabs you can see, and jump down into the water.
Be careful of those white "ropes." They're really barnacles, creatures that drag you into their mouths with their huge tongues. Being dragged by a barnacle is an instant death. If you do get dragged, start shooting or stabbing the barnacle as fast as you can before you reach the top.
Dodge the barnacles in the water and swim toward the three openings at the end of the canal. Keep following the passage until you get to a ladder, where a nearby health station lies. Use it if you have to, and then walk through the small blue door. Kill any remaining headcrabs you missed (be careful not to fall into the canal) and then step through the next small blue door. Follow the thin hallway until you get to another door. This leads to the crate shipping room, which is a room you will most likely hate.
Crate Shipping Room
Climb the ladder nearby and then climb the next ladder to get to the highest platform. I recommend saving the game at this point. Walk up to the crate that's at the very left of the platform and jump on it. Original Half-Life and PS2 players have it easy, while Source players have it twice as hard. In the Source version, the crates will actually move when you land on them, so you must be extremely careful when you go from crate to crate.
Now jump on the next crate that's directly left to the one you're on now. Now do a diagonal jump to the one that's to the right and in front of you. Now jump down onto the crate that's directly in front of the one you're on now. I also recommend to save it at this point. The hardest jump is next. To the very right is a crate that's by itself. Don't jump, just walk forward and you should land on the crate no problem. Now just walk forward to the one on the left, and then jump down onto the platform.
All done. Go through the blue door up ahead and just follow the hallway until you get to a ladder. Climb up, and go through the next door. Take a right until you get to an elevator, which you activate by pressing the nearby switch. Go in the elevator, press the switch inside, and you'll descend into the next level, Office Complex.