Where to start, oh my. There were the first two rolls of 120 I put through my newly acquired Pentax 645 that were loaded backwards, so the backing paper was blocking the film. That was bad.
Or the first two rolls I put through my newly acquired Mamiya RB67, and didn't notice the mirror was set to lock up, so the only thing that was happening when I tripped the shutter was the mirror slapping up; no exposure was being made. That was bad. A lot of fine shots with amazing skies were lost to the wind that day, I can tell you.
Another great day was when I was developing a roll of 35mm, pouring in the developer, and glanced away to get my finger on the timer switch, looked back and saw that the developer was overflowing the tank because it apparently airlocked...and of course, not enough developer got into the tank. That was bad.