So it might be a function of the "Update files for external changes" option... I'm not sure. The files it lost didn't have any changes, other than attempting to move them via the Rename From Properties tool. In fact, those particular files had been tagged for months and I never got around to moving them from the "incoming drive" to the "media drive".
It also doesn't quite fit that the new entries that Otto imported for the 12 files that got lost pointed to the files in the old location (before the move), and were broken (of course since the Rename process did actually move them).
Either way, I've disabled that option for now (I don't really need it anyway -- I use MC for essentially all my tagging). I'll have to see if it happens again. For now, I've already finished tagging and moving most of my files so I don't have a big batch to use it on anymore. I did test it with a slightly smaller batch of 25 files and it worked just fine. That could have been a fluke though!
When I have more time, I'll test it more thoroughly.
Oh, and Larry, I certainly have files other than AVIs. I suspect Alex is right and that it would apply to lots of file types. I just don't have the time to spend figuring it out tonight unfortunately!