Probably.
I rebooted PC, and the issue has gone now.
I've been able to replicate a variant of this issue documented with log at 0016130. The key in that case was:
1) MM must be obscured by another app
2) MM must be playing music
3) User must type the SPACEBAR in the app that is obscuring MM at the time that playback transitions from one track to another.
In my case there was no music running ... although I had tried playing a video file with MM for the first time earlier in the day, and I had trouble stopping it ... so maybe "something" was playing
There would have been so transitions, because it was a single video file.
MM5 was obscured by another app.
I didn't try other keys. I noticed it with the Delete key. Luckily MM displays a confirmation message before auctioning a deletion request.
I think that the active application was the Gmail client inside the Windows browser ... I started up Notepad, and the leakage to MM5 continued.
It is the 1st time that I have noticed the problem.