So, this "question" is really oddly phrased, because it also apparently can be just a thing that you saw happen to one of your gaming buddies, but I'll keep it in the context of at the game table. I also assume that it could be worded as "Most Memorable Moment", and that's kinda how I've approached this.
I have to say that out of all my years of gaming, the Eberron game I ran from September '08 to December '09 has had the most consistent stream of memorable characters, scenes, and interactions I've witnessed. One of these instances is the forerunner in my mind for the craziest thing I've seen at the table that has happened during a game.
The party had been beset by devils and demons for quite some time, often taking on vrocks and bone devils (osyluths). During a confrontation with one of the bone devils, at a time when the party was low on resources and actually had to fight hard, Taylor decided his Artificer's Iron Defender, a small construct built to resemble a dog (and not much more powerful than the animal it favors), was going to attack the thing.
In this campaign, I was using the critical hit charts I posted long ago, which had effects ranging from negligible to deadly. Taylor rolled for his construct and scored a natural 20. If I remember correctly, the thing could only hit on a 19 or 20, but nevertheless was able to confirm the critical hit, then subsequently rolled head on the hit location and then high enough on the effect chart that the iron defender ripped the bone devil's head clean off. Everyone at the table roared with excitement and laughter.
It was just one of those moments where the chips are down, you're using all your tools - in this case, a construct with barely a chance to damage this thing - and then, sudden triumph. We still talk about it, because it was just that awesome.