Linebacker Dwayne Rudd of the Cleveland Browns
Rudd cost the Browns a victory by taking off his helmet and flinging it in celebration as time expired and the Browns were about to defeat the Kansas City Chiefs 39-37. Of course, what Rudd did is an automatic 15-yard unsportsmanlike conduct penalty. Since a game cannot end on a defensive penalty, the 15 yard penalty moved the Chiefs into field goal position. Morten Andersen calmly converted the field goal opportunity, giving the Chiefs a 40-39 victory.
If Rudd still has a job tomorrow, he may do well to head for the nearest Dumbasses Anonymous meeting. (Hi. My name's Dwayne, and I'm a real dumbass....)