The very first day of my very first job (I was 16 years old....office environment) the lady that was in charge of me had a very small light mark on the back of her pants.
I had only been in her presence like 15 minutes (she was very stern/authoritarian like with me, letting me know up front that they run a tight ship, etc I felt like I was being chewed out and I hadn't even DONE anything yet! After having gotten to know her later I realize it was because they had bad experiences with student workers in the past and wanted to make sure it was clear up front) and she had gotten up to go get something (and was coming right back) when a friend of hers pointed it out to her as she was walking out and the lady turned around and gave me this evil LOOK and said something like oh thanks for telling me, trying to make it sound joking but the look on her face was not a joking one. Maybe I should have jumped up and exclaimed "HEY YOUR BUTT!!!" but I was young, unsure of what I should/shouldn't say and very, very nervous. If anything, her friends/people she had been with all morning should have told her.
However, ever since then, I make it a point to let people know (discreetly) if they even so much have a crumb on the corner of their mouth.