In The Exorcist, when Linda Blair starts to levitate from the bed, you can occasionally see the wires.
In Poltergeist, when the kids' bedroom closet opens up to become a huge monstrous maw, you can see the wire holding the worm-like tongue.
I wish that whenever they re-release films on DVD, producers would try to digitally clean-up those gaffes (unless the director intended to have those things to be detected?).