.This should be fairly obvious, but it's always good to keep your VCR's remote handy while watching a tape. If you ever hear an odd "metallic" grinding while playing a cassette, usually accompanied with picture disturbances, hit STOP and eject the thing immediately. A "fixed" eaten tape is usually the culprit and the slack and crinkles from such damage can chip or snap off a VCR's very delicate video heads. If either happens, you'll be looking for a replacement deck.
This can occur with both dirty old ex-rentals and pristine tapes--basically any previously opened VHS (that you didn't personally unseal). If you find a damaged tape and still want to keep it, it's best to label it somehow for future reference. If you still want to watch it, I'd only recommend so if you have a VCR that can fast forward without queuing the tape up against the spinning drum to move beyond the ruined section. Below is a mint looking copy of MEDIA's Death Wish 4 that played with a champ until about an hour in. Thankfully no harm came from this one.