I think that if you see this only in some movies that these movies
are partialy corrupted it happens when you download them from
net and you know there might be some problems in connection or
transfer or something.

I saw some of these also some have this only occasionaly but
especially Ogg embeded avis were corruted all the time
it looks like little blocks are dancing all around having impossible colors
and place in some scenes like somebodys shadow got teared and
distorted and stays in place where it shuldn't be anymore.

Hope it helped you.