What’s the funniest thing you’ve done as a DM to a group of players?

Comment from discussion Hi Mike!
Big fan. What’s the meanest/funniest thing you’ve done as a DM to a group of players? Not that anything DM’s do could be considered mean, they’re great. I unconditionally love them and would never want to upset them. All DM’s are powerful and good looking. Fact.
Also, what’s the meanest/funniest thing you’ve done as a player to another player or DM?
Meanest thing as a DM: It’s small, but it’s the first thing that comes to mind.
3e Eberron game, the characters are fighting their way through a frost giant outpost high in the mountains. The giants send a pack of dire wolves out to rush the PCs.
The halfling player comes up with a fiendish plan. He decides to leap on the back of a dire wolf and attack. The wolves can’t bite at him while he’s there, he declares. It would be crazy unrealistic for them to risk biting their packmate. Plus, the giants would clearly avoid throwing boulders at the halfling and risk squashing their pet.
I agree with his cogent points.
On the giants’ next turn, they use a free action to whistle and ready their actions to ends that are inscrutable to the players.
Halfling player is perplexed.
On the wolf’s next turn, it runs back to its master in response to the whistle.
Giant #1 pulls the halfling off the wolf’s back. Giants #2-6 pummel the crap out of him.
That was the last time he tried riding a giant wolf.
Funniest/meanest thing done to another player – buried a rogue alive, not realizing that the ring of regeneration he stole from the fighter was going to revive him in a few hours.AMA: Mike Mearls, Co-Designer of D&D 5, Head of D&D R&D
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Running Basic D&D for our in-office campaign – pt.2

What is going to happen to all the playtest material that isn’t in Xanathar’s?