I didn't personally see this, but heard this from several sources about a mom of a player who we played with one season:
Cussing out the coaches and the umps during a game and then yanking off the team shirt and walking topless out of the ballpark may rank up there pretty high in the no-no list!!
One of my pet peeve's is the parent you hear the loudest; and its always only for his/her kid; even when the play is not involving them. One parent in one of our last games was cheering for his kid in the on deck circle while the other at bat was still going on.
I also can't stand non-coach parents giving technical instruction to their kid while pitching or batting during games. Do that in practice! not the game!