I bought what appeared to be a MISB set from Walmart, only to get it home and find out someone had opened it, filled it with trash and a bunch of knock-off brand bricks, then resealed it. My first indication that something was wrong was that it sounded a bit off, though not enough to really rouse my suspicion, especially since it was still MISB, with now visible sign of tampering. What really tipped me off, was when I tried to open it. The glue they used to reseal the box was much stronger than Lego uses.
I had to fight with Walmart about returning it, but eventually got my money back.
Steve