Wishes - How do you handle them?
Looking over this thread, it occurs to me (a) I've been GMing for twenty-five years and (b) in that time I've done a "wish" sequence exactly once, and that was as a plot point discussed in advance with the player of the wishing PC. Actually, I could stretch it to twice, with the same circumstances. They were both in The Watch House (one was Matthew making everyone happy and the other was Milli and Ziggy seeing each other's point of view... so to speak) and neither of them worked as intended for the PCs in question.
Were I to involve a wish-granting sequence without knowing what the players would do in advance, I'd have to (a) trust the players and (b) I'd generally keep things in the spirit, unless I'd established we were dealing with an Overly Literal Wish already.
Wishes going badly are certainly valid plot hooks - Vengeance Demons grant wishes in Buffy and if they're not bad already they sure go that way - but so are wishes going well, albeit in a lighter-hearted kind of story.
A smart player could twist a wish into a game-wrecking hyperpower. But hopefully a good player would know game-wrecking hyperpower isn't fun for very long.
So I wish you good games.