The craziest thing I every saw happen during a D&D session was also the funniest in play action to take place in my old gaming group. We were on one of our marathon gaming sessions on a Friday or Saturday night. I do not remember exactly how many of us were playing but it was 3 or 4 players in addition to the DM. I can not even recall if we were playing a published adventure or something the DM had put together.
What I do remember is that we had been playing most of the night and had fought our way through hordes of enemies and the hit points were getting low. At this point, we were being a bit more cautious in our actions. We arrived at a place with a close door and decided not to just rush in but to listen at the door to see if we could tell if anything was in there.
The first character stepped up to the door and placed the side of his face against the door and heard nothing. We were sure that there was someone or something behind the door so the next player stepped up and heard nothing also. If my memory serves correctly, I believe the DM kept saying, "You failed to hear anything" if one of us failed so we kept trying. All of the players attempted to listen at the door and heard nothing.
Eventually, the turn cycle back around to the first player to attempt the listen at door action. He thought for a moment and said, "My horse is going to listen at the door!". Some light giggles were heard as the DM rolled the dice behind his screen and said, "The horse succeeds!". As luck would have it, we knew something was going on behind the door but none of our characters spoke "horse" or whatever you want to call it. At this point, the session was pretty much over for the night. Everybody seemed to get a case of the late night laughs brought on by being up way after bedtime and drinking a ton of pop. I do not even remember if we went into the room then or later but I will always remember that one guys horse listening at the door.