ooo a fun topic!
I tried to make these two stories quick, but it's really hard..
I've never seen a fight, but I saw one almost start:
The group I was GMing for captured a female NPC and wanted to question her. They sequestered her until they could figure out what to do. One of the players (a knight) said that they would not let any of the other PCs hurt her to get information. One of the players questioned her in private, and so the player and myself went into another room so the other players would not hear the questions. That player was an odd person and his character gave the NPC prisoner a hicky. Did I mention he was an odd person? Anyways, the knight (stranger than the other player) went in next, and he found out about the strange mark on her. I decided to go with it and the prisoner told the knight that the other character hit her (2ed days - no Bluff rolls or Sense Motive rolls) The knight player went back into the room, grabbed the other player by the collar, held up his fist and said "If you ever do that again, I'll kill you." The other player said "Let go of my shirt" in a flat tone, and the knight player did..
And during all of this, one of the players not involved in the altercation had invited their girlfriend to the session. She assumed that physical threats were normal for the group.
-Story 2-
We have a player who will get into these moods where he comes over and looks for a way to get someone to get mad at him. He clearly does not do it because he finds it fun or funny. It's really strange. He will also throw temper tantrums, too, but these are really rare.
This past Saturday, the guy called up and said he was on his way. He called at the time that we would normally have started, but admitted that he had fallen asleep. He worked for 13 hours earler that day, doing physical labor from 4am to 5pm. No one was blaming him for falling asleep, or being late. No big deal, I just told him to come on over when he was ready since he said that he wanted to play. He then called me up about 20 minutes later, yelling at me that he was an a-hole and that he shouldn't be playing because he can't make it on time (he used to have a problem with punctuality and was ashamed when people pointed this out, but got over that bad habit some time ago). He was yelling like he was mad at me, but never accused me of anything. It was strange.
I found out later that he had raged like this before, and recently broke his cell phone in one of these rages.