During a mix using digital vinyl control, the first deck suddenly stopped playing. I did not take any action at the time that might have triggered this. Mixxx did not crash, it was just a sudden halt to playback. The WSpinny timecode signal overlay was frozen. Everything else continued to work. Going in to preferences and looking at vinyl preferences, the signal widget there was still working (!). Re-applying the vinyl settings fixed the problem (!). There were no errors in the log whatsoever.
This is a very weird bug and I doubt I'll ever see it again, but I'm logging this bug report in case it comes up again.