While chess is a pretty well regulated, there is still a level of trust between the players that makes each game run smoothly. Most of the time we don't even realise this, unless out opponent does something really weird. Over the years I have been queried about such topics as 'eating a full dinner at the board', 'clicking of pens', or 'my opponent is standing behind me', which fall under the heading of irritations, rather than changing the actual outcome of the game.
However I do occasionally hear of behaviour that does cross that line. Most recently I received a second hand report of an incident from the Zimbabwe Open. In a third round game, one player was ahead on time, and after playing his move, went for a stroll around the playing hall. When he returned 3 minutes later, he found that his opponent was claiming a win on time. He was somewhat surprised as he had more than 3 minutes on his clock when he left, and it was his opponent was short of time. Apparently his opponent had replied to his move, and then simply moved the clock to the other side of the board (reversing the displays). Despite protestations, the arbiter upheld the claim, and as a result the aggrieved player withdrew from the tournament.
(NB This somewhat reminiscent of the Reshevsky v Denker game from 1942, where Reshevsky lost on time, but the arbiter picked up the clock from behind, and after turning it around, decided it was a loss for Denker!)