I've just run into the exact same problem as everyone else here. The only things that have changed recently are (in order of least recent to most recent):
- I added an Apple Magic Trackpad
- I swapped out my Logitech mouse for a genuine Apple Magic Mouse
- We updated our old Parallels (6 or 7) to the latest version
We absolutely need the new Parallels since we'll be upgrading to Mavericks soon, but I'll try something with the other two and see if they might be the cause.
