Going to the wrong endzone with the Huttball.
Oh goodness. Yes, this. I felt like such a moron, I thought I was doing so well, too...
Not quite as bad but there was also the time I respawned at the goal line in Huttball and saw an enemy Jedi jumping into the pit and heading toward the goal line. "There must be someone stealthed up here!" I thought, so I start running around the edge of the pit looking for a stealther... only to be lept at by the ball carrier for an easy score.
I can only imagine how red my face was.