For me, my biggest "screw-up" was during a WEG Star Wars game. One of my players wanted to play a character who had heightened senses, like bullet time. Every time he was in combat he got a +1D to Dexterity, +1D Perception, and +1D to Strength.
Needless to say, he was a combat monster. Anything that could cause him problems was enough to kill any other PC.
I was in high school at that time, so I didn't know any better >_<
-EF