Overworking The Ears
You should never allow your ears to be over worked in the process of mastering music. You should always begin a project with a fresh pair of ears so you don't get a thing called sound fatigue. This is an actual temporary loss of your full range of hearing which is why it is important to rest the ears and don't attempt to master music at loud volumes.
After all of the meticulous work of the mastering process in completed, then the engineer can play the song with it's full power to reference the final outcome. But doing this too much during the task and you lose those auditory thresholds where you cannot not hear key elements that need to be adjusted. It's almost like trying to enjoy a song after a fire cracker has gone off in close proximity to your ears causing immediate ringing. When that happens the only cure is to rest your ears from any further damage in that case.
For every job there is the right tool an that tool needs to be in the best shape and condition to perform at it's best. Need I say more that when you use any part of your body as a tool, the same care an preparation also applies.