I think I ran into this on two shows. The files were photo-jpeg, encoded using compressor, and using presets I had made. The first show I ran into this, I wasnt able to figure out what was going on, the second show I was able to track it down to a specific file, and re-encoding it (oddly enough using the same preset) fixed it.
It doesnt seem to always happen, as I did batches of encoding where the files were all made using the sme program, with the same settings, etc, and it only happened on one file in the batch.

The probelm I would see, so that if anyone else runs into this they know, is that the file will freeze playback, but all other parameters are still controllable via DMX, i.e. color, intensity, position, etc. It really threw a wrench into my day when it happened, as I figured it was something else that had gone wrong with the server.

Jason