I haven't figured out what triggers it, but sometimes (often after waking from hibernation??) my Ctrl key "sticks." The physical key isn't stuck, but the software thinks it is. I can't type non-Ctrl keys or do almost anything.
I used to see this occasionally with my previous (Vista) laptop. When that happened, I could often clear it up by hitting the Windows key, or Shift, or some other locking key. That doesn't seem to work with Windows 7. If it gets into this state, I have to reboot to clear it.
What causes this, and how can I prevent it?