Unfortunately (or luckily), i can't reproduce the problem at the moment. (Removed MM3 completely and did some registry cleanup)
Ifound a few heavily corrupted "mp4" files, that crashed Mediainfo and/or did not play correctly with any player (no audio or worse).
Maybe by chance the few "destroyed" files happened to be of that kind.
Still a little strange though. The files had mostly ~200MB and MM seems to have replaced the whole "content" (mostly already garbage maybe) with the 2kb tags content (?).

I see what task all you brilliant guys have shouldered, handling this jungle of formats, codecs etc. ... .

I think, your files are not real MP4, they just have MP4 extension. We plan to add some wrong extension detection to avoid this. Please, check them with MediaInfo tool, as described here, and let me know the result, thanks.

Tracked as http://www.ventismedia.com/mantis/view.php?id=7178

Sorry, for sounding dramatic, but this is the first really destructive bug I encountered so far:
Trying to change the property "type" for a bunch of mp4 video files to "video" results in complete destruction*:
They end up as 2kb garbage files.

A similar behaviour was already described here:
http://www.mediamonkey.com/forum/viewto ... kb#p280751
and here:
http://www.mediamonkey.com/forum/viewto ... kb#p147537

*Luckily i was able to kill MM soon enough with the taskmanager to avoid greater damage