A couple of days ago, ChessBase reported that a player in the Dutch Chess League (a team captain no less) has been caught using Pocket Fritz during a match. His game was declared lost and he has now been banned for the remainder of the season and the next two years.
This reminds me to two things:-
(1) I once, during a London League match, saw Stewart Reuben recording his moves on a PDA rather than writing them down. I've no idea what program he was using and I'm not suggesting at all that he was using a chess engine (I've no doubt he wasn't) but it did strike me as strange at the time. I was very surprised his opponent did not object to this - I certainly would have done.
(2) During a chess tournament many years ago, a friend of mine was playing a junior player who kept disappearing when it wasn't his move. Deep into the middle game, my friend made a move, pressed the clock and went off in search of the junior to tell him it was his turn.
My friend found his opponent in a room nearby. He was standing next to a board with their position on it. In fact the move my friend had just played was already on this board. This room was full of a bunch of other young chess players and ... one of the arbiters of the tournament.