There have been plenty of crazy things I've witnessed in over three decades of gaming - so many that I couldn't possibly pick out one. Hastur slain in a single stroke. A hasted (?) vulture lion killed with a single thought. The fate of entire adventuring parties hinging on a single, desperate roll.
I wasn't there, but one time the party stumbled onto the back door into the Pit of the Oracle. And yes, that would be the treasure room.
But the craziest moment? That would be a series of moments scattered over the years, and they were some very animated "discussions" about the rules. Dino arguing about his character being able to tie/untie knots in midair whilst falling. Craig trying to crawl around and drink a potion (think "fly casually"). Clerics trying to reach downed party members as they are about to roll over from -9 to -10 and perish. A thief trying to gain backstab positioning on dragons, demons, or any number of highly intelligent and magical creatures. You get the picture.