I do know someone who does quite often.
Via another great resource, Get Rich Slowly, is a piece on why you should check your recpeipts:
- As in our recent trip to IKEA, you might be overcharged.
- You might also be undercharged, and have to point out errors to the checker. (I’ve found that sometimes a store won’t even bother to fix a small error if it’s in the customer’s favor.)
- You might be charged for a completely different item. Computerized databases speed transactions, but they’re not foolproof.
- Sale prices may not register. This is actually fairly common, especially at the supermarket. If you’re after a particularly good deal, be sure you’re paying what you expected.
I've seen mistakes caught by doing this, and there are several Google'able research papers on how grocery stores often make mistakes on the list and charged price.