There are times I throw tough ones at my players to see how they handle them, and one example was my very first scenario after coming back to GMing in 2003 after an eight year hiatus.

So there's this Bad Cult, doing bad things in its badness.  It's got a bunch of young girls in cages, preparing to sacrifice them to its Bad God.  Underground temple, only one access point, and effectively impossible for the party to assault, with eight or so mercs hired by the Bad Cult lingering around.  It's in the po' part of town, the wrong side of the Boulevard.  The local guard commander was on the take, the PCs' political protector was an admiral taking his squadron out to sea in the run-up to a war, and just about no one in power would've given a damn ... just another bunch of scum from the Mariners' Quarter, who cares, they breed like rabbits anyway, right?

So they notice the press gang coming by their tavern (the political protection ran so far as to exempt it from the press), and the dwarf fighter scritches his head and says waitaminnit.  And goes over to the lieutenant running the press, and hands him some coin, and points to the entrance to the tunnel, and whispers some.

And in the wake of the press gang gleefully dragging most of the mercs off to their ships, the party launches its assault ...

So, what whacky stunts have your groups pulled?

Running Hoard of the Dragon Queen for the first time it was a long running outside the box.

Right off on encountering the town under siege the players start sneaking in rather than fighting.
Then they blunder in the smoke and shadows into a gang of kobolds and the Nox the dragonborn promptly claims hes working with them and that their dragonborn leader is really a traitor. Which the kobolds end up believing.

Later the group infiltraits the cult and from then on all their interactions with the cult are them pretending to be members. The moments of actually hindering the cult have been when either no-one is looking or they are sure no ones going to live to tell.

Even when the ruse was seen through they wheeled and dealed through it.

And the oft mentioned otter incident where my wizard character was killed in an ambush and the groups druid used a scroll to cast reincarnate. And I came back as an otter. For the rest of the session I pretended to just be the groups pet or something and the enemy party later ambushed us and was making a point to get the otter too after wiping out the rest of my group. Why? Because they thought I was the wizards familliar. So they had me cornered and were rather surprised, and shortly dead, when said otter hit them with a fireball at point blank.

And a Star Frontiers campaign where the players were ambushed on their way to deliver cargo. Being chased they first cobbled together a makeshift torpedo and actually got it to hit against all odds. And when one of the ships grappled them they decided to try and rotating the ship to swing it into another. And lastly they jettisoned an engine into the surviving ship.

Still one of my favourite sessions DMing Star Frontiers.

Heh, your otter anecdote reminds of me of my favorite stunt as a player.

I was playing a rogue in a VD&D campaign, and rather than a stereotypical thief=burglar, I was more a stealth-and-misdirection guy.  So I had gimmicks like fishhooks on blackened strings, lumps of sugar smeared with ash (those produce huge clouds of billowing smoke for the volume) ... a bombard, which is in effect a very teensy oboe that makes a great loud shrill piercing sound and is perfect for the "pipings of insane demons" sounds.  Things like that.  Also I was lucky enough to have my friend Rich Meyer (of Gamelords fame) in the group, who was an excellent, inventive and quickwitted player.

I've long forgotten precise details of the adventure, it being nearly thirty years ago now, but it involved getting past a guardian into the Abandoned Underground Temple, and incidentally getting an object he had with him to boot.  Said guardian was seemingly very tough, and we weren't murderhobo types anyway, and inclined to find some way to let us all get out of it with a whole skin.  So I suggest we all sit down and have a bite and a drink.  Guardian agrees, albeit wanting to use his own wine for fear of shenanigans.  Fair enough, of course, sir, feel free, indeed I'll prove my trust by drinking your wine, if you don't mind, sir.  And I murmur to Rich under my breath, "Play along with whatever I do."  Bless his heart, he did.

So I start this running patter, somewhat sing-song: "We're here to find the Rare Mountain Peacock, which legend has shines like sapphires and rubies, isn't that right?" (looks over to Rich)

Rich: "Like sapphires and rubies, that's right ..."

Me: "And half as tall as a man, with wings out to HERE, and eyes flashing like the heat lightning on the horizon at midsummer ..." (looks over to Rich)

Rich: "I dunno, more orange than red, don't you think?"

I talked increasingly fast, and Rich kept up, and we were firing back and forth, faster and faster, not faltering.  We were sitting about 10' apart, and soon everyone's eyes (the DM's included) were following back and forth between us like spectators at a tennis match ...

Me (everyone turning to look at me): "... but before we tried to catch the Rare Mountain Peacock, we needed a bottle of the finest Jholaix wine, because legend says it'll take nothing but from a man's hand ..."

Rich (everyone turning to look at him): "So thank the gods we brought two bottles, just in case!"

Me (diving right in, and dropping the accent I used for Daerion): "And that's when I slip the sleeping draught I've been palming into the Guardian's goblet."

And all the eyes turned back to me, startled ... and after a long pause, people break into uproarious laughter.  And I smile sweetly at the DM, her jaw dropped half a foot, and ask her if I really need to roll against sleight of hand.  (I don't recall that she made me.)

In the last DCC session, my players turned a random encounter with an illicit-dolphin salesman into creating an explosive-dolphin-launcher for use in their upcoming Death Race.