Something stupidly easy to catch, like putting a different TV in the box.

Fraud

Years and years ago I had the misfortune of knowing someone that bragged about how he'd buy a video card at best buy, swap a dead card in it, then redo the shrink wrap at his job (an indie computer place) before returning it. There's lots of banal scummy scams you can do like that.

I specifically know of people who abuse the Best Buy membership alongside the return policy to essentially rent equipment for periods of time.