Here's a screenshot:

I think it has something to do with the desktop animations, but not sure. I don't care about this since I can easily remove it by closing the app which opened it or restarting the DE, but I've seen this:
http://community.linuxmint.com/idea/view/2330
This looks similar to what happens on my system so I think there's a bug.
I'm not requesting for help, just informing about the bug, if any additional data is needed to solve it, just tell me and I'll post it.









