I got a random deletion again. This time without doing a multi-select to trigger it, which I thought was the key.
But I was doing about a 700MB download, which is reputed to be iffy in beta11 according to the FAQ.

The exact error is:
The length of playlist BLAH did not match the actual number of children detected.

Interestingly one of the *other* errors (the decoder.cpp error) went away.on its own.
I was all ready to reinstall beta11 and hijack to see if I could make it go away. Then I checked and it was gone.
It seems to intermittently reappear from time to time. (Ummm, I guess that *is* the definition of "intermittent.") but I haven't determined if it is due to the contents of the database (i.e., if syncing new files makes it go away) or if it is more hardware related and it just appears on some boots and not others.
Now that I'm out of room and won't be downloading any more tunes, I can check into it. (Yes, I need a new hard drive, but I'm not about to violate my warranty until I make sure this is not a hardware issue.)