I don't have photographic evidence because this happened way too fast, and I hope I'm not the only one that thinks it's disgusting.
a few days ago, LGA-ATL I was upgraded to F. After I sat down, in comes this guy in a t-shirt, a bit out of breath. Obviously, he was running since he was sweating. He stows his luggage, sits down, and the FA comes around to hand out the hot towels to everyone. I use mine and then see him wipe his hands with his towel, his face, and then proceeded to shove the towel into his shirt to wipe his UNDERARMS.
Of course, I then wondered if Delta reuses these towels and if the one in my hands had been on someone's underarms before. Yes, I know the towels are sterilized, but still...