A couple of ones from our last campaign. These won't be as funny as they were when they happened, but you'll get the gist.
We had a minotaur in the party, and during one game, someone wondered what language minotaurs spoke. Turns out, it's "giant", but we assumed at the time that it was just "minotaur". Thing was, the minotaur was a barbarian, and had no ranks in any language skills. So, the player playing the minotaur merely assumed that everyone else was speaking minotaur the entire time! "they all speak minotaur real good!"
In the same game, the party rescued a village of lizardman who'd been trapped in a demiplane that had been their home for generations. They found a portal out, and led them back to a place that was infested with Slaad, and other nasty creatures. Most of the party was more or less indifferent towards their fate, but a couple of the players were relatively concerned that they would need assistance getting back to their ancestral home (which, incidentally, they plundered, and killed the king, but that never bothered to mention that to them). And the party sorcerer (who as LG) said, "At least two of the party members have your best interest at heart!" That still cracks me up to this day.
In other campaign, the GM had given the sorcerer a staff of snakes (it turns into a snake when you throw it). This player was all about style so he decided that his character just wouldn't use such a device, and got rid of it (I don't exactly remember what he did with it). Several games into the campaign later, we meet this wizard in combat, and the sorcerer declares that he's flinging his staff at it, forgetting that he had abandoned that staff, and now had a very mundane staff in his possession.
The guy playing across from him (in mocking fashion) said "behold!", makes a motion like he's tossing the staff and says, "clank, clank, clank". I still laugh at that one.
The best one though was a Spelljammer game I ran in college. I'd borrowed the books from a friend and decided I was going to run a game in Spelljammer. It was high level. I don't remember exactly what happened, but here's the scene:
The party is on a ship, in space, being attacked by another ship with mindflayers, and umber hulks. At some point, someone casts something that creates a lot dirt, which is now swirling around the ship because of the artificial gravity. One of the mindflayers is an illusionist, which promptly summons an illusion of a "fire lich" (from one of the splat monster books) - it's basically a flying, flaming skull. So, the party wizard freaks out and is fighting that (because he's read those books, and thinks he knows what it is!), and there were other freaky things going on. My friend just absolutely lost it. We had to stop playing because he was having trouble breathing. Needless to say we never played Spelljammer after that, and I still have a hard time taking the setting seriously.