When I had a job at the photo-developing counter of a pharmacy, the film was sometimes developed incorrectly.  If this occurred, we did not charge our customers. 

Once a man came to pick up his pictures, which were marked $0.00, indicating that not one of them had turned out right.

The customer asked to see the photos, and when I noticed the odd splotches of color all over the pictures, I apologized profusely.

"Oh, no, these are fine," he said happily.  "I'm a microbiologist, and these are my bacteria cultures."