My husband and I collect comic books. The closest shop is about 40 miles away, but a local book store stocks News stand covers. These are usually worth a bit more than the other first prints for the book, so we'll go in there and pick up any that are still in good condition- the book store is rough on their comics, so more often than not, we leave without getting anything.
One day we went in and found a really nice copy of one comic and decided to pick it up. We took it up to the cashier and she rang us up, put the book in the bag, and grabbed our receipt.
She then proceeded to put the receipt on top of the bag to show up the link to a survey at the bottom. It was one of those "rate the service" kind of things...
And she took her pen...
And said "make sure you give me a 10!"
And drew the number 10 and circled it on top of the comic book.
Which effectively left a giant 10 indent on the front of the book.
Comic book grading is incredibly strict...and her drawing on the book had severely reduced the value of the book because it had put a dent/crease in the cover
We still go in there, but if we see her working, we walk right back out.