When I add tags to .mp4 files, window media player (or photo gallery) can no longer replay the file.  I am not sure if it is the "People tags" or the "Descriptive tags" that caused this but shortly after (within a few minutes of) renaming and adding tags to a bunch of .mp4 videos the files became unplayable.

Thankfully once I had done my tagging I could see windows going through my files one by one switching the thumbnail from a movie thumbnail to a generic mp4 thumbnail, this is the only reason I cottoned on to what was happening.

If anyone (ie QA or dev) can pick this up I can supply uncorrupted and corrupted versions of a couple of the files.

Needless to say this could be a disaster for some people if they run into it - fortunately I hadn't removed the originals from the device on which they were shot.

By the way - Microsoft really needs a mechanism to report issues like this!