There was the roll of film I had a few weeks ago at my 1-hour job that was covered in a weird, oily substance. No way we were going to run it through, and another tech pulled it out in the light just to show the customer. Especially strange was that the whole roll was oily, but the emulsion was not sticking to the base when it rolled together. They never told us what it was, nor how it got in the film canister and all through the roll. Perhaps they dropped it in a cold deep-fryer?
Then there was the lady who wanted us to digitally decrease the size of her butt in a treasured photo. We re-did it for her three times, to the point where there the photo was very obviously altered but she refused to believe that the photo was an accurate depiction of the size of her rump. Ma'am, if you read this, your ass really is that big! Accept it, and move on.
Every once in a while we get someone who wants a rectangular print from a square negative and wonders why we cut the feet off. My answer? "We thought you'd rather lose the feet than the head." (more politely put, of course.)
Of course, as Bob noted lab techs are not immune to mistakes or pure stupidity either. One time we received a roll to be printed as 3.5x5 and my colleague didn't read the 3. and printed the whole roll as 5x5. Or the time I opened a nearly 400 foot long roll of 6" Fuji CA paper in room light, and took a good 30 seconds to figure out why the paper was blue, or visible at all.