I experienced this problem again, under slightly different conditions, so I thought I'd add the info here.

This time, an album playlist was deleted, and I got the warning about the parent playlist (the artist) having an inconsistent number of children. However, the songs on this album were not deleted and I was able to recreate the album playlist by copying from the Soup view.

Here's the important difference about triggering the bug.
I wasn't editing Wendy Flags this time; I was correcting an artist name misspelling. And I definitely was NOT doing the edit in a soup view, since those edits seem to be flaky and/or non-sticky. So the bug seems to be related to doing a multi-select, and then editing a field to be applied en masse. Afterward, an unrelated playlist was deleted. Perhaps some sort of overrun problem in the attribute modification code?

Also, here is some weird stuff from my boot log in case it has any bearing on this (doubtful). But let me know if I should be concerned about this for other reasons.

First, some disk detection problems:

Tried to mount /dev/hda4 as reiserfs but got error 19
Mounting second music partition
Tried to mount /dev/hdc4 but got error 6
Error mounting partitions (possibly already mounted)
Remounting first music partition read-only
Remounting second music partition read-only

Then an error at entrance to the player:
! mp3_decoder.cpp :1049:Failed to find valid sync after seeking to offset 2267
281, error=0xc0044000

The later error reappears if I drop into the shell and then "exit" back to the player.

Neither of these problems seem to adversely impact the normal operation of the player, however. As far as I know, they've always been there.