This one is easy, and although it's more than 15 years ago now, I still feel pain just thinking about it:
One of my customers, a travel agent organising tours and events for incoming tourists etc. to Thailand, wanted some spectacular photos of their 30-40 strong staff with Bangkok as a backdrop. So they rented or borrowed the helicopter platform on top of one of the city's luxury hotels and stopped all their activities for a full day to have the photos done.
The chosen photographer was me, and knowing that this was a once-in-a-lifetime event, I brought all the gear that I had available at the time, Olympus OM-3, OM-2S and an OM-10, all of which were in regular use. Long story short: the OM-10 didn't advance the film, but since it was the least used camera (for the all important single portraits), I didn't notice until I rewinded the film after coming home. The metering of the OM-2S, my main camera, was suddenly way off. The OM-3 worked very well, but I had just acquired that camera second hand, and wasn't very comfortable with it, so only used it for the large group shots with the 21mm.
So I ended up with some very few usable photos and a day wasted for 40 people. Ironically, I had a Canon A95 with me that I had bought a few days earlier. I took a few shots with it, mostly to show off the new digital gadget. I would obviously have been much better off doing the whole job with the Canon, but I didn't trust that camera yet, and my whole body screamed in agony just by the thought of doing a paid job with a point-and-shoot.
It was after this that I reluctantly bought my first digital SLR, the Fuji S3.
Every time I see a "The photographer ruined my wedding" headline, I think about that day, knowing very well that anybody can screw up. "There ain't no such thing as too much backup, too much checking, too much quality control".