Super Mario World/Red Switch Palace

Walkthrough

 * Right as the level starts, a Koopa will enter a yellow Koopa shell and turn invincible. Luckily, he is blocked for now so you're safe.
 * You have two choices. You can either step on the P-Switch, which turns the brown blocks into coins and frees the invincible Koopa shell. Jump over him and run to the end; the Koopa shell will follow you and destroy all of his buddies on the way, earning you lots of points and a 1-up at the end. If you don't get the 1-up, run back to the left and you may find the last Koopa you need to get it.  Then just wait until the P-Switch expires and the invincible shell is trapped between a pair of brown blocks again.
 * If you want to play it safe, however, you can ignore the P-Switch and just jump over all the Koopas on your way to the pipe.
 * On the other side of the pipe, step on the red switch to enable all red switch blocks in the game. You'll also be given an opportunity to save the game.