I once heard about a producer who would immediately end a mix session as soon as someone uttered the words, "Turn up the cymbals."
The point was, this was a measure of ear fatigue. The upper midrange frequencies (where the cymbals live) are the first to go when your ears get tired from mixing too long. So once someone needed the cymbals turned up, he knew it was time to call it a day.
This leads me to wonder about eye fatigue. Is it time to turn off the computer when you misread “Avid” as “Rabid?”