A friend of mine studying medicine once told me this story.
Apparently one day there was a lab where all the students were
learning how to identify various cells. As samples they were
using tissue scraped from the inside of the mouth.
One girl was having terrible difficulties figuring out what kind of cell
she was seeing under her microscope--eventually she called over the
teaching assistant to identify it.
He came over, smirked, and exclaimed (loud enough for everyone to hear)
"Oh wow! That's a sperm cell!"
She was somewhat more careful after that experience....