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. These are my bacteria cultures."