I haven't made a lot of processing mistakes like fixing before developing and have only once opened a tank lid (although slammed it back so quickly I didn't notice any damage). Even when I have multiple unlabeled trays of print developer out, I never forget which one is which. But a few noteworthy moments of brilliance:
1) Exposed brand-new paper to light. THREE TIMES. Although the damage has shown to be minimal thus far, every time it was something special. One was a 500' roll of 4" RC that I had waited for a long time to order from Freestyle because I knew it would come with 10 tons of styrofoam peanuts, and wanted to add things to the order to make it worthwhile. Somehow I thought the brown paper bag was an outer bag and there would be a black bag inside. Nope, the brown paper bag was coated in black and I exposed part of the roll. One was a pack of 11x14 MCC, which I'm sure you all know is frequently sold out...had left the bag open and turned the lights on. Oh, and then there was the free 100-sheet box of J&C Nuance a kind APUG member gave me...on the first day I was using it I made the MCC mistake...again.
2) Had a sudden jet-lagged panic moment while taking a few shots with my Fujica Half--was pretty sure I had Neopan 1600 in it but the ISO was still set on 100 for autoexposure. Since at one point recently I'd also had 160S in there, I decided "not to risk it" and rewind, check, and forward to the same frame again later. Except I guess I let go of the rewind button briefly enough for something in the mechanism to stop working. And even though I was SURE from the loose way the knob turned that the film was no longer rewinding, I decided to open the camera immediately instead of waiting until I got home to my dark bag. On the subway, under bright fluorescent lights. Completely ruined and yeah, it was NP1600 all along.
3) Forgot I was developing 120. That one was covered above. I ended up just agitating more and it was OK but not great.
4) Just about 20 minutes ago, I had on my sink a brown 1L bottle that I was soaking with dishsoap next to a bottle of DD-X. I know they look and feel nothing alike. However, I was watching TV--er, multitasking, I mean--and instead of dumping out the soapy water in the brown bottle, I dumped out a bunch of DD-X.