Firefox v33.0.1 (this will have changed since I started seeing the error a couple of months ago) and Windows 7.
Regarding debug logs, I turned on the requested options in preferences and was able to cause the bug to occur by mashing keys, but no log was generated. However, the session log does...
I'm using Firefox and have experienced this issue. I first noticed the problem when I started using a Logitech wireless keyboard, but I'd imagine that's a coincidence. Only ever happens while I'm typing, but I'm almost positive it's not a result of an accidental key press.