Rudeness (CSC)

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: Randy: "Herb, wait until it's your turn."
: Herb: "Oh, stuff it, dickbreath, my character's doing something more important than yours right now anyway."
: Rita: "Guys, where's the last beer I left in the fridge? I can't find it."
: Herb: "Hey, URRRRP!!, I'm sorry, OK? How was I to know that was your last bottle."
Grumpiness, grouchiness, bad manners, interrupting players during their turns, eating food that wasn't intended to be shared—whatever—the end result is that one player (or more) is acting in a way that insults or annoys his fellow players and the DM. This type of behavior encompasses a lot, from the guy who openly insults someone else (even a joke can hurt) to the fellow who presumes that the food in the host's fridge was provided for his benefit. Most people have enough tact to keep unpleasantries to themselves or to behave within the boundaries of common courtesy. When they don't, tension and bad feelings rapidly increase.
When it does happen, the DM needs to respond quickly and gently, but firmly. Ask the players to be civil to each other. Stress the need for team play. If necessary, set down some common courtesy rules.
Enforce a standard order of play, who goes first, regardless of where people are sitting or what their characters are doing.