Downloaded films stop working

I've downloaded a few films from BitTorrent sites. Most are fine but
one or two are acting oddly. Initial download was fine. Checked again
and the film played ok on Win Media Player then next time I tried it
crashed the player and froze. The weird Dr Watson error came up. I've
checked for viruses but nothing doing. Why would a film work for a
while and then afterward crash on me? Could there be some code in it
to allow you to view once or twice then it explodes on you?