Making a hash of it all
@Neoc - With regards to your last paragraph; I believe people do actually try and do that, which explains why I've seen a fair few videos of 'films / TV / music videos / etc.' which have been horizontally flipped so everything is mirrored.
I imagine they use multiple hashes over the most likely tricks these 'cunning' souls are likely to perform. But as with everything, some will slip through the net.