I was playing a Dragonborn Berserker and our party was battling our way up a narrow ridge. We were right at a choke point where the monsters couldn't see us unless they were on the single tile either behind the choke point or in the choke point.
Being a berserker, I charged right in... then was promptly down and out within one turn. Since I wasn't quite dead yet, I managed to take up a single square on the grid that no one could pass unless I was healed.
My party, instead of healing me, used my body as a barricade to launch ranged attacks over. My near dead body helped win the battle!
Love it! One of our most hilarious moments was due to a bad interpretation of opportunity attack rules. Specifically, we thought that someone rising from prone would provoke op attacks.
So our cleric went down, and was immediately surrounded by enemies who had knockdown attacks. He kept trying to get up, and would get punted back down every single time. It was good for the party, because he kept them busy--as DM, how was I not going to exploit this?--but he very nearly died.
Eventually he decided to just stay put, and attacked from prone.
We were just dungeon diving and had come across a feather with unknown magical abilities. With no mage in the group or any person capable of identifying it, we simply continued on in the dungeon in hopes of possibly identifying it later.
As we got deeper in to the dungeon, we came across a group of gargoyles that were, quite frankly, destroying us. We had basically run out of ideas and were close to being defeated. Then our friend had the bright idea to try and use the feather on a gargoyle, not knowing what would happen.
As it turned out, the feather was for acceleration and speed and flotation, based on the DM's die roll. The gargoyle got the flotation and flew away in the sky like a balloon.
Let's see, I've got plenty of great combat moments from all sorts of games, but if we are going with D&D, I'd have to say that the funniest moment I've seen was because of a character named Sergio.
I was always obsessed with the classic stereotype of the spanish suave character, much like Antonio Banderas. He was suave, arrogant, and a true swordsman... so Sergio was created in the form of a Swashbuckler prestige class. And the best part, other than his pure arrogant nature was that Sergio never referred to himself in anything but the third person.
So, the group was trudging through a swamp when they were ambushed by a wild group of.. something. I don't recall what. Either way, the enemies had the group surrounded, the battle was intense and Sergio was in true form. That was until one of the enemies cast Fairy Fire on him and lit him up like a Christmas tree. So, in the middle of combat, Sergio stops fighting, sheathes his blade and steps out of combat to the horror of his teammates. And of course, they quickly started asking in a loud and angered fashion what he was doing. Sergio's response was simple, "They make Sergio glow, so people can see Sergio better. Sergio likes."
Funniest thing was, the enemies left Sergio alone since he was no longer a threat. The IC banter after that was priceless and I have never been able to live it down, but hey.... you gotta really play the character, right?
During the adventure path Age of Worms we were in the gladiator adventure. There are rules for how it is bad form to injure gladiators too much or kill them, but one player just kept rolling ridiculously high at the last moments and killing his opponents. Finally, they make it to the final battle against Alric (?) who is the reigning champion. I decide he calls the player out for solo combat with some speech like, "I'll show you what it means to be a real champion."
Initiative is rolled. Player rolls a 19. Player charges. Player power attacks for all. Player rolls a 20. Player confirms crit. Player deals enough damage to kill Alric outright in a single swing. We all stood and cheered in one of the geekiest actions I've seen.
Probably a PC being knocked out of a cave by a monster (a Chimera, I think) - not a fatal blow in itself, but it knocked him right across a mountain path, down a precipice and to his doom (after a failed Dex check). Hilarious, until he failed his resurrection survival check during a Raise Dead...