Way back in the day (1920's? 1930's?) this advice was given and I've seen it trotted out countless times since:
"It often happens that a player carries out a deep and complicated calculation, but fails to spot something elementary right at the first move. In order to avoid such gross blunders, the Soviet master B. Blumenfeld made this recommendation:-
When you have finished your calculations, write down the move you have decided upon on the score sheet. Then examine the position for a short time 'through the eyes of a patzer'. Ask whether you have left a mate in one on, or left a piece or a pawn to be taken. Only when you have convinced yourself that there is no immediate catastrophe for you should you make the planned move."
-- KOTOV (in Play like a Grandmaster, I believe)
I think "Blumenfeld's Rule" was gospel for decades, which explains the widespread practice, but FIDE have disallowed it so it can't be used in any tournament that uses FIDE rules.
I remember being astonished on reading an article somewhere when Abby Marshall explained a poor performance in a scholastic tournament by the fact her mp3 player broke so she had to borrow someone else's to listen to during play, and didn't like the music so much
It certainly seems to be allowed by the rules of those competitions (so not cheating) but I can't remember ever playing in one where it would be ok. Then again, mp3 players weren't around when I was a junior...
The rule on "you must not distract your opponent" is one I'm more likely to want enforced. I haven't yet encountered a player who likes to cough a lot during every game while their opponent is thinking, although I know of a few.