missing part when ripping a title

I observed with several DVDs that after ripping a title a part of it is
missing. The missing scene was there when previewing the title in 
dvd::rip before ripping! The scene is already absent in the ripped vob 
file, i.e. before transcoding. I also found that in these case the rest 
of the (now too short) movie is filled up with a repition of a scene 
from within the movie. Is this a bug of dvd::rip or transcode or a 
consequence of some kind of copy protection?

Raphael