A word to describe today: frustrating.
I duplicated a physics object to make some collision filtering easier to manage, and all seemed well with the world when I did some testing. So, I finished adding some juicy visual and audio effects to the level 1 demo. However, the player now has some serious bugs which I can only attribute to the earlier duplication of the physics object... Deleting the object makes no difference and reverting back to a system backup does not precede the object's duplication. Luckily I zipped all game files to an archive yesterday and all of that works as it should.
Lesson 1: make more frequent zip archives.
Lesson 2: don't copy physics objects unless you really have no alternative (ie don't do it).
Lesson 3: there's no way to submit this as a bug report, so the problem might manifest itself again....
The bright side: having to re-do 6 hours of work is better than having to start the whole project again....!!