The engineer controlling a mix can be the least reliable judge of how it sounds, and it has nothing to do with skill. Spend enough time listening to the same balance and your reference for normal starts to change. The harshness you would have caught in the first minute becomes the sound you expect to hear by the second hour.
Years ago, I was mixing a show for a national touring act and I could not get the sound check to come together. Every change I made was making the mix worse. The artist was getting frustrated and so was I. One of my friends, who is a very senior and experienced engineer, was in town and stopped by just to say hello. He walked to the console, didn't say a word, and made a dozen small moves and gave me a bit of a grin. The mix fell into place like magic. It sounded the way it should have 30 minutes earlier. After the sound check I asked Bill where I had gone wrong. He told me that I hadn't gone wrong anywhere. I needed fresh ears. My ears had gone stale and I had stopped hearing problems that a fresh set of ears would catch instantly.
Bill's advice has stuck with me. When I mix now, I step away for a few minutes and come back, because the break rests my ears and the problems I could not hear become obvious. I do the same for system tuning. After a couple hours of reference tracks I leave for 15 minutes and let my hearing rest before I trust another decision.
The person who knows a room best may also be the person most accustomed to its faults. If you own a room, or a mix you have lived in all day, your confidence is the symptom. Build the rest in before you sign off on the sound.