What's your favourite "I have to do what you told me, but not in the way you wanted" story?

The wish spell. Genies. Deals with devils. Even lower-ranked abilities like suggestion-type spells, depending on how they're used. It's a classic trope of RPGs that someone will use some sort of magic or magic-like ability to force someone else to do or believe something, or cause something to happen, but for that thing to not necessarily go in the way the person who did it intended.

What's your favourite story in a game you've been in where something like this happened?