Last year I visited a well-known restuarant very near the pond in Breckenridge, CO. The meal was nice (but not fantastic, considering the price), the service was good. We tipped 15%, I wrote $103.00 on the receipt and duplicated the figures exactly onto my copy.

Later I checked my credit card statement and found it was $108.00 but didn't have time to return since we had to leave. I called the restaurant when I got home (in the UK) and the manager said their copy had the grand total at $108.00, but agreed that the total and tip came to $103.00, but would refund the $5 "if that's what you intended to tip".

I didn't accuse their staff outright, but I said I was very sure it was written as correct 103 and could explain why his read 108. I didn't ask to see a copy, but my 3's have a flat top and cannot be turned into an 8 easily, so it should be obvious to him just looking at it.

I did read a method for easily detecting this sort of thing, I think it was where you just repeat the last two digits reversed as the decimals, eg. in my case I would have writen 103.30, then it appearing as 108.30 would be easy to spot without having to remember or find the receipts. (This assumes the staff don't bother to alter the cents)