Keyboard keys seem to be stuck even when the keyboard is unplugged

While not doing anything unusual in particular, I used a keyboard shortcut (left Windows key + X) to start Amarok playing a stream (the main Amarok window was hidden; only the tray icon was showing). Thereafter the system acted as if those keys were stuck in: Amarok periodically (every few seconds) restarted the stream; I couldn't use the keyboard to type.

I unplugged the keyboard (a USB one) from the computer, but the system still acted as if the keys were stuck in or being repeatedly pressed.

Sys Rq + R,S,E,I,U,B did work however, after which reboot and upon logging in, Gnome settings manager (or something similarly named) failed to start—a warning dialogue was shown; the panels launched upon dismissing it, with the theme reset to Gnome default. This error didn't happen (and the theme settings were back to normal) after logging out and back in.

I have no definite or even approximate steps to reproduce this, and didn't try to.