Back in the day when I used to attend dance parties, I would do my hair in a high Mohawk style and then spray it with a UV hair spray to stand out under the UV lights used throughout the place.
My mate, who shaved his head and only had a goatee saw me doing it and said he wanted to spray it on his goatee. So off he went and did exactly that. Fast forward a few hours and the dance party had some night markets with UV lights to highlight some body piercing jewellery. I thought a new tongue barbell would be great if it glowed in the UV, so we went to check them out. My goatee wearing mate stuck his head into the light and lit up like a Christmas tree. We all started laughing at him, so he pulled his head out, promptly switching his head light off and said "What?".
We were all laughing so hard we finally managed to get him to stick his head in again. He stuck it in and once again the UV lit him up. Once we found a mirror he saw himself and panicked, and wanted to wash it off. We convinced him, truthfully or otherwise, that washing it off would only make it worse, or spread it. He decided to leave it on.
We didn't realise to the extent of how funny the situation was until he led us into one area, and the back of his head lit up with a single big UV hand print on the back of his head. He was so easy to spot no matter where we went from then on, and if we lost him, we just had to ask if anyone had seen the guy with the hand print on his head.
He never did it again.