I have a pair of Ray-Ban polarized sunglasses (so not some cheap drugstore sunglasses, and they are made in Italy not China) that I got two years ago. I just got back from a vacation, and I noticed that one of the lenses has some sort of damage along the bottom:
On the inside of the lens, you can see some of the coating has started to come off:
What happened? I never dropped them. When they're not on my face or hanging from my shirt collar, I always put them inside a hard case. I never leave them sitting unprotected in a car. If I ever leave them in a car, it will be for less than an hour, and they will be inside of the case inside of my backpack. Sometimes I will clean them using Zeiss lens wipes. Also, I don't wear them every day - I only wear them on trips, so they have not been subject to excessive wear.
I have heard of something called "crazing" that can affect lenses that are subject to excessive heat - could that be what happened here? The only other thing I can think of is sweat, but I don't know if perspiration can damage lenses.