As you can read in this post:
Problem with displaying VobSub subtitles
I have stumbled on such a file. But for me it works OK, using old subtitle engine, and in preferences -> subtitles -> vobsub I have enabled VobSub suport and load subtitle into memory.
It may be related to using ffdshow as decoder (I am not sure). I am using CoreAVC for x264 files not ffdshow.