Sounds like bad encoding to me, but it could be almost anything.
If I turn up levels to high in Coaster i can get that pretty easy.

If you are fairly sure its nothing on your end and you can replicate the problem at the same point in the file each time I would guess it is a bad file.

The only thing you can do is note the size of that file and try to get another one that isn't that size so you avoid getting the same thing again.