Okay, so the rules are simple. One poster makes a wish, then the next poster comes up with a way to turn that wish bad. Then they wish again, and so on. Here's an example:

Poster 1: I wish that the entire world was made of chocolate.

Poster 2: The sun melts the chocolate and turns it into a big chocolate puddle floating through space.

Alright! I'll start...

I wish that I could fly.