I think my own worst experience was after my first (and only, so far) trip to Iceland in the summer of 1974. I shot about 30 rolls of Kodachrome in addition to a bunch of black-and-white (35mm and 4x5). After I got back I sent all 30 rolls of K-chrome to Kodak for processing. When the slides came back it was clear that something had gotten into the production line the day my film was processed. About half of the pictures on every single roll had streaks of some gunk across the surface.
After several mail exchanges with Kodak, they agreed to try to clean the slides (without admitting that it could have happened at their end). They did succeed in cleaning the worst off maybe 75% of the slides, and, of course, they gave me 30 rolls of film to replace the film that was damaged.
Fortunately, amost all of the best pictures were either untouched by the goo or were adequately cleaned by them. Since that day I have never sent more than three rolls of film to any one lab on any one day.
A couple of my friends had a bad experience a few years ago. They were both view camera types, one using an 8x10" and the other an 11x14" (yes it was big -- just imagine the file size from a digital back for that thing). Anyway, they had been out on a shoot together and were packing up. The driver got into the SUV they had come in to turn it around, without realizing that the other guy's 11x14 was still on a tripod behind it, and backed over it. As you can imagine, they both felt pretty miserable about it.