In the latest podcast from the Chicken Chess Club, Laurent Fressinet admitted that in his latest tournament, the French rapid and blitz championships, he made an illegal move. He asked the other two podcasters when they last made an illegal move. Jan Gustafsson said that his last illegal move was when he was 17 (so about 1996) and rated about 2400. Peter Heine Nielsen couldn't remember.
So, how often do strong master level players (2400+) make illegal moves?