[vlc] decryption fails in middle of movie

[Stuart McGraw]

Hello,

Please forgive what might be a dumb question, but
I am very new to VLC and computer video stuff in general.
I backup my commercial DVDs by copying the files from the
DVD to my hard disk and for most DVDs, I can play these
backup copies fine with VLC.  However a couple movies will 
play fine though most of the movie but somewhere in the 
last 1/3 or so, playback will become corrupted, as though 
the dvd is no longer being decrypted (i.e. looks like what 
one sees when playing an encrypted dvd without libdvdcss).  
This happens on the same movies at the same place with 
vlc-0.99 (IIRC) thru vlc-1.03 on Linux and Windows.

A while ago I tried to use some video tools to debug it
further and think that the decryption failure occurred
in the movie at the start of the last .vob file, but I
forgot how I figured that out.  

Can someone explain why this happens, and better, how to
stop it from happening?  Thanks for any help as this is 
happening on my two favorite movies!