Back when I was in high school, I played chess with a group of other students. One of the students would pick on me sometimes. Once, we decided to play and he cornered me into a forced mate in two. Before the threat turned into the final execution however, I swept my arm across the chessboard vehemently, blasting the pieces across the room. Needless to say, he got very mad. I spontaneously kept doing this for a few more games (to my sole entertainment).
Curt von Bardeleben is known as the opponent who never returned. After move 25 in the Battle of Hastings, Steinitz's opponent mysteriously disappeared:
At this point, Steinitz's opponent left the tournament hall and never reappeared. By these offensive means he hoped to deprive Steinitz of 'a piece of immortality', ... - Art of Attack, V. Vukovic. pp 21
What are some of the most unsportsmanlike manners witnessed in over-the-board chess play?