Some moments from my D&D game that I ran for a year:
After running through a spontaneous dungeon crawl, I was giving the players the list of loot that they had gotten. On this list was "thirty-seven pieces of art, to be determined later" because I was too tired to do it then. I forgot that one of the PCs had a fixation on dwarves. (Female elf, btw.) "Can one of the pieces of art be a dwarven thong?" Me: *gnk*
*sigh* "Sure, whatever." She kept that d**n thing for the entirety of the campaign.
On a separate occasion, the party had tracked down where a group of evil cultists was setting up a gate, because they were starting an army of gibbering monstrosities from beyond the bounds of the rational universe. (Byakhees, for the CoC fans among us.) My players found the clearing where the gate would appear, staked it out one night, and determined that the cultists could only come through one at a time. There were many of them, and they didn't know what kinds of fell powers the cultists had. So they spent the entirety of the next day digging a pitfall where the gate would appear, lining it with spikes, and disguising it crudely. (You couldn't see through the gate.) Night comes, the portal appears, the first cultist steps through, falls into the pit... and screams something incoherent about a trap. Ten seconds later, the portal vanishes. Party's Rogue: "You mean that they can hear what happens on the other side of the gate?!
Party's sorcerer: "Ah... I knew that." All: "THEN WHY DIDN'T YOU CAST A SILENCE SPELL, IDIOT?!"
Same campaign, but I was taking a break from the GMing, and was playing that night. We were trying to kill a greater vampire lord. Who has, by the way, mind-controlled our fireball-happy sorcerer. So, after twenty minutes of dodging fireballs while trying to get up onto the roof vamp boy was on, the paladin and the druid fall back and discuss something. Paladin: "What would happen if we dumped forty-seven gallons of holy water on him?" GM: "Ah... screaming and dying, I would imagine. Why? Where are you going to get forty-seven gallons of holy water, and how are you going to get it up there?"
*Points at druid* "She creates water approximately four feet above his head, and I simultaneously bless it. At this level, she can create forty-seven gallons a day, and I can bless a small lake." GM: "..." (This did not, in fact, completely kill the vampire, but it did hurt him severely.)
Later, tracking the same vampire to his lair, we come across an alchemical laboratory. The paladin picks up a bottle, sniffs it, and takes a gulp of the contents. He promptly belches a massive fireball. So we keep that, and my fighter picks up a similar looking bottle for later use. So we find the vamp's lair. Paladin kicks the door down, and my fighter immediately fires two crossbow bolts into the now very startled vampire. The halfling monk gets between us, and kicks him four or five times in the head. Then the paladin uses up the rest of his firebreath potion, breathing fire all over the vampire. I take my bottle, and throw it at the wall behind the vampire's coffin, and it turns out to be a concentrated fireball. (The monk, during all of this, is sublimely unconcerned, because we've hit the point where area affect spells cease to matter to him.) Then the monk throws a stake he picked up earlier... and triple criticals, sending it straight through the vamp's heart and pinning him to the wall. Whole combat: seven seconds, game time. The GM was rather stunned.
In a modern game I was in, the party had to talk to a boy who was being stalked by evil bad things. We sent our two most respectable people: a screwball doctor, and a frighteningly intelligent eight-year old. The kid was scared, and the doctor kept making things worse. So the eight-year old tasered the kid, had the doctor drag him down to our car, and tied him up. When the kid came to, needless to say, this did not incline him to trust us. This resulted in the line "Does anyone besides me see the problem with the antisocial, drinking-shooting-smoking gun-running former gang member being the only one who managed to diplomatically handle that situation?"