I am running Ableton Live music software (with L4 also open), and I wondered why <Ctrl+Alt+B> (show file browser) wasn't working in Ableton.
It turns out that L4 steals the keystrokes even when another application has focus.
Having hit <Ctrl+Alt+B> in Live, the Copy Bible Verses window appears in L4.
This is 100% reproducible, and I'm sure it happened with a different keystroke the other day, but I didn't twig at the time.