I once found this damage on my headphones on both sides, it is unclear what caused it, and the continued uses causes the damage to expand. I’m seriously concerned as I’m sensitive to such issues. The headphones are less than a year old.
Solution: Glasses have been the problem, read one of the many replies below for more details.


Could also try Goo Gone. It’s specifically meant for removing adhesives, though I don’t think the basic version works on CA glues.