when I was a teenager my mother used to cut my hair. I would usually wear underpants, but for some reason I was naked. I was about 15yrs old and was starting to mature. Whilst having the haircut I got a massive erection, not because I was turned on or thinking about sex, it was just one of those spontaneous hardons. I should have said I needed the toilet or something but I just sat there and she carried on and neither of us said a word! Very embarrasing.
Another time was when I was at boarding school, same sort of age (14 or 15). They made us wake up and go straight to the showers so everyone's nightmare was waking up with a morning glory and being forced to go to the showers with just a towel (and the showers were open)