So you grant the wish of the previous poster, but somehow corrupt it to make it undesirable.
Example:
Person 1: I wish I had a million bucks.
Person 2: Your wish has been granted, and one million male deer will arrive in your yard shortly.
Then after you corrupt the previous poster's wish, make your own wish for the next poster to corrupt!

I'll start us off...
I wish I had a Cloak of Invisibilty.