A few months back I was playing some great levels (I think by
OmegaDyne) and he was doing a neat thing that I didn't know was possible. It turns out that you can overlay any number of pipes, and if they overlap, the most recently linked one will be the one that Mario uses when he enters the pipe. This allows you to
Create several entrance pipes that all lead to the same exit. If you go back into the exit, the entrance pipe you end up at will be the one you last linked.
Have a pipe that, if you re-enter it, takes you somewhere other than where you were when you entered it the first time.
My latest stage,
Fort Hammer, does both of these things. The first area has a two pipes that take you to two different rooms, but then both lead to the same checkpoint afterward. And at the end of the stage, you can re-enter a pipe in order to access a secret bonus area.