According to Mental Floss, there's a real reason why Costco checks your receipt at the door.
Here's the scenario - you're at Costco, just got through the checkout, and heading out the door when an employee stops you to check your receipt. Why??
Turns out, it's for a very good reason...and in your favor.
Costco has their employees make sure that customers were charged the correct prices for the items and that they were properly scanned. Some former employees admit it happens quite a bit.
But don't try leaving the store without that mark they put on your receipt - members agree to abide by this rule in the Costco membership application terms.