Snakers
When playing online, you no doubt come across snakers at some point. There are two ways to beat snakers: learn to snake or drive the race of your life. If someone is an expert at snaking it will be vitually impossible to even get close to beating them, even with all the best weapons on the game.
Cheats
Some people just can't play fair, and they will use cheat cartridges to allow them various power-ups that are impossible unless you cheat. There are various ways that these people cheat, such as giving themselves infinite power-ups (such as stars, mushrooms, or Bullet Bills). If someone has chosen to do this, there's not much you can do apart from cheating yourself, or just carrying on regardless with a warm sense of pride in heart.
Rogue racers
Play online for long enough and you encounter a "rogue" racer - someone who just plays to make everyone elses life a misery, possibly due to sexual frustration or maybe because they were dropped on their head as a child; who knows. Sometimes these rogues will cause this game to crash pretty much straight away. so you have to turn off your DS, but other times they will victimise one racer just for the fun of it. The offending player will usually sit near a cluster of item boxes, not caring about being in last place, and trying to pickup a Star item. Then when the other racers go past he/she will use the Star and collide with a racer, and then reverse and hit them again, and again, and again - sometimes upto 6 or 7 times in a few seconds. Sadly there is nothing much you can do if someone choses to be like this.
Disconnecting
Disconnecting is when a player switches off their DS or just stops playing for 30 seconds, during a race. It can also happen accidentally due to a loss of internet connection but this seems less common. It usually happens when someone is in last place and feels they have no chance of winning, so they give up. If only 2 players are racing and one disconnects, the game ends and none of the wins picked up in that match will count. This is especially annoying if it happens to you when you are leading in the final race. Again, there is nothing you can really do, other than possibly tricking weaker racers into thinking teyhave a chance of winning and then snatching victory at last minute when they don't have time to disconnect.