Infinite Wish Loop
"I will grant you three wishes, but no wishing for more wishes."
"Gosh, I wish I could wish for more wishes."
"... fuck."
Role-playing games with powerful wizards usually have a spell called "Wish" or similar at the top tier of spellcasting, and it's supposed to be the wildcard of do-as-thou-wilt magical effect. But any DM knows the problem with giving players a blank check: they're going to actually try to cash it in. If the game wasn't written by a noob, the spell description for the Wish spell will have specific limits on what can be wished for, including "No wishing for more wishes!" and "No wishing for more beings who grant you wishes" in order to prevent cheesing via infinite wish loops.
...Ha ha, screw that. The Wish spell is supposed to be an "I win" button, and we're going to win the fuck out of this game. This page should be a list of how to get nigh-limitless uses of the Wish-type spell effect in games.