I've had this happen to me on Chrome. It was definitely a browser bug, because restarting the browser resolved it. I first noticed it when I was working on a web page with complicated CSS rules for 'cursor'. If I had to guess, it has to do with loading custom .cur or .gif files as the cursor, since it didn't seem to happen unless I tried to do that. However, it was very sporadic and I could never get a reproducible minimal test case.