Normally it's cat hair, brush hairs or a variety of other kinds of annoying fuzzy things that end up in my paintings. Most of the time I get them out before the paint dries.
This time though, one strand of my hair landed in the gesso, unnoticed (look closely, it's on the left). No chance of removing this one, so I will just pretend it is added character. At least it fell into a very graceful line.
I'd wear a hair net when I gesso, but frankly, I am just WAY too cool for that. Heh.