Not afraid, but something bizarre did happen to me once. I used to have an account on some other art site that I abandoned shortly. Then I was browsing a newer art community one day and saw someone with an avatar that I definitely recognized. I messaged them, and found out someone was "selling" those avatars on Second Life (of all places?!?). I'm talking, like, a five year old design, drawn in a squiggly fashion by mouse in Photoshop and uploaded someplace. Five years later, someone claims it on Second Life and sells it to others?
If they'd done it with ANY other picture I had there, I wouldn't mind, but the thing that bugs me is that I can still use that design now. When I made games in The Games Factory, I used it as a sort of company logo, or signature, for my games, and I have a pretty large decal lying around in my closet with that logo integrated in it that I managed to get for free a couple of years ago. If I ever started a company or something, that would definitely be my first logotype. So that was a little frustrating to see someone "selling" that very design.
I do feel frustration when someone thinks they can do whatever they want with something I've made. But it's nothing I worry about, and something that's very easy to deal with anyhow.