Cross Posted from the "DM Hazing" thread - The best one I ever pulled as a player was this:
We were up against this cult of shapeshifters and our party of 3 (my halfling rogue, a human barbarian and elven cleric) were in this underground passageway about to sneak into one of their hideouts. Suddenly I say, "I think I've got this all figured out." and I Sneak Attack the barbarian. I do great damage and then win initiative whereupon I Sneak Attack him again, killing him (we were low level so not a lot of HP's). The GM is looking at me like I'm totally insane and so is the cleric's player. The barbarian's player is flat out pissed.
I'm saying, "Don't you see! HE's one of THEM! Watch, he'll shift back to his normal form now that he's dead!" We watched the barb's body. "Nothing" says the GM.
"Did it take a while for them to shift back to normal form after we killed the other shapeshifters?"
"Nope."
"Are you sure?"
"Yep."
"Well, I think that there's one thing that you're not considering."
"What's that," says the GM. You can read the anguish on his face about how I've derailed the campaign.
"It's April first."
Me and the barbarian's player burst out laughing since we'd engineered the whole thing. The GM and the other player both breathed a big sigh of relief and we retconned back to the start of the session. Everybody agreed that, in retrospect, it was one of the best 15 minutes we'd ever wasted in a game.
As a DM, I'm not entirely sure what you could do, particularly since I'm not familiar with the module you're running. I do recall an April Fools Day joke adventure from Dragon Magazine many years ago. It was called "NOGARD":
So basically you have the party get somehow magically transported to this "alternate plane". They find themselves standing in the middle of this vast, endless plain that is made of some kind of slightly spongy material. There are no features in sight. That's because there are none. NOGARD is just an endless, flat expanse of nothing. The funny part is to watch what the PC's attempt as they try and figure out the "trick" to the place. After all, there must be a reason why they're there, right? They'll try and dig through the spongy ground or fly way up in the air to see some kind of landmark or something else entirely. They'll get increasingly creative, then frustrated, then flat out desperate as they become sure that they're missing some hint that you've dropped. Somewhere in there you explain that this is an April Fools joke.
That might work or not. Hopefully some better suggestions will be along later in the thread.
