What are some of the most unsportsmanlike conducts witnessed in over-the-board play? 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?
Here are some examples of unsporting behaviour I have heard of before:


Kicking opponent under the table (from Josh Waitzkin in the Chessmaster tutorials)
(Added for interest) Running down the clock in correspondence chess with the aim of waiting for an opponent to die of old age (this shouldn't be a major issue under the ICCF, as games with deceased players are now cancelled or substitutes are called, but they could be misrecorded as a loss on time)
Using an iPhone for analysis (hat tip to @SmallChess)