1) Allowing the Game Master to use the suggestions of a player or players to screw them over in game.
2) Giving the Dungeon Master knowledge to help defeat or hurt the player.
Wizard: "I am going to cast mage ar... wait... Technically, my spell book got ruined after we got thrown into the River."
Cleric: "Damn it! Why did you have Jeff Yourself like that!"
DM: *Grins* "Roll a percentile to see how many pages were ruined."