On Illegal moves

In yesterdays post I referred to the Zhao - Rujevic game from the 1999/2000 Australian Championship. This game was interesting in that during the game Zhao played an illegal move (Rook jumps Pawn) which was not noticed by either player. It was however noticed by a spectator (John Pascoe) who did the right thing by informing the Chief Arbiter (Me actually) about what he had seen. As the game was in the Australian Championship (rather than say an under 10 tournament), and neither player had said anything, I (incorrectly) dismissed the claim. The truth was revealed at the end of the game, to the shock of both players, when a replay confirmed what John Pascoe had seen.
Interestingly I have heard about this happening more often than I would normally expect. It is as though both players 'expect' that the game will follow the rules, and therefore don't scrutinise the legality of every move. The diagrammed position is a very recent example of this. Two moves previously Black's pawn on a5 stood on a7, and White's pawn stood on b5. Black then played a7-a5 to which White replied b5xa6ep. However instead of removing the a pawn, he confidently lifted the b pawn from the board. Black, who I believe was at the board when this happened, didn't sense anything was amiss, and the game continued on as though nothing untoward had happened. It was only a couple of days later that White realised what he had done, and again when told, Black was genuinely surprised that he had also missed this.