As I see it, t'ain't no such thing as a 'stupid' mistake. We all make mistakes, and if one admits to them and learns from them, the stupid factor disappears and it turns into a learning experience.
A few pointers I have learned by default (or fault) over many years...
If you habitually leave paper boxes open in the darkroom (as I do, don't we all?), print yourself a large sign saying something like PAPER BOX or IS PAPER BOX OPEN? or SHUT PAPER BOX, laminate it,and put it up on the wall. Twenty years ago I gave in to temptation and paid A$100 for a the customised enlarging paper safe, a strange European box-like thing that never worked right for me, I was forever frustrated at having to yank it open and shove it shut for only one sheet of printing paper. Eventually I gave it away to a friend who also hated it. I believe it ended up in recycling.
Putting a wine glass (ideally full) on top of the enlarging paper box, also works for me. Your mileage here will differ, you may prefer beer or moonshine. Horses' courses here.
Another of my recurring problems is I forget to check the settings on my Nikkormats or exposure meters before a new session. Cloudy day scenes are then shot at EI 100 and tropical beach shots at EI 400, the stuff of major disasters. Now I reset cameras and meters at the end of the shoot. Haven't burnt out a beach shot since.
Occasionally stuff-ups work for you. In the '70s I went to Bali for my first-ever holiday there, out of Sydney, Australia, where I had just relocated to live. At the last minute and racing out the door for the taxi to the airport, I opened the fridge door and took a metal can with (so I thought) my rolls of 120 Kodacolor and Fujicolor negative films. Once in Bali, I took out the can to load up the Rolleiflex, and realised I had taken the container with Kodak Tri-X 120 rolls in it. Roll film being all but unobtainable at that time (I recall finding a roll of well-cooked Agfacolor on a dusty shelf of a small toko in a place called Klungkung in Bali, yes, I bought it), I had to use the TXB&W P, well filtered with orange, green and even red from my set of Rollei filters. Nowadays I use plastic food containers for my roll films, so I see what lies within without having to open it. Amazingly, all those early Bali images sold well as stock for years, and now and then one still sells as visual nostalgia. Nobody else can give away a Bali photo now, but this isn't the point of my story, is it?
As for film developing disasters, I learned long ago that the old Leica two-bath developer (or Thornton's two bath, idealy Ansel Adams mix) is best for me to salvage a reasonably (two stops or less) overexposed (best) or underexposed (more risky). Such negatives are often infuriating to enlarge with long exposure times, fiddly developing with strange chemicals added to the Dektol, and increased red wine consumption (see my earlier point), but with a certain non-analog process I know I could be slowly strangled, tied to a pole, staked in the lower guts and incinerated to a crisp by the APUG Thought Control Gestapo here if I name, so I won't, good results can quite easily be achieved.
In photography or darkroom work as if all of life, one should try to attain that exalted state of not making the same mistake more than twice, once in error, then one more time to fine-tune the learning from it process. Mistakes are not fun, but for me a lesson learned the hard way is (usually) a lesson well learned.
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