Never hang out with an audio mixer. They point out some many noises in day-to-day life that I have just tuned out or never even noticed. Once they point it out though, I can't not hear it. To be fair, I'm that way about video and compression pointing out to people how shitting cable quality is. I'm a TV sales rep's worst nightmare.
There was one movie I was watching and I noticed the sound of a (propeller) plane flying overhead. I wouldn't normally have noticed this, except I was wearing headphones. I couldn't help but wonder if they noticed. and if so - did someone said - "screw this, we're not reshooting the scene."
And sure enough, one of the overhead fluorescent lights had an electrical humming coming from it.
And it became hard to study from that moment forward.