Ask me, I know. So where do I start? Let's go with darkroom disasters.
Use ancient film. Says he who has 100+ rolls of old emulsions and several cans of long-expired Kodak B&W in his fridge. At least it's being temperature-controlled. And worth heaps on Ebay. Very tempted to...
Ignore time-temperature calculations. Ditto, use developer, stop bath or water, fixer all at different temperatures. The patina on your negatives will make all your images look like museum mosaics, this is guaranteed.
Rely on stand developing with highly diluted developers. I call this One Size Fits All formula, but the problem is usually the results end up being the wrong size with the contrast all over the place, usually down.
Use Dektol full strength on your Tri-X or HP5. Guaranteed ink black results. Bloody murder for scanning.
(I did similarly in the 1980s with Rodinal at max strength for Tri-X. Got six rolls of valuable negatives from Bali too grainy tand contrasty to be used for anything other than Zone VI filters.)
Use 20 year old Kodak Rapid Fixer. This one undid me recently. I bought the stuff in 2010 and kept it in a box in our garage until a few months ago. Fortunately, refining in fresh fixer saved the films. Never again.
Mix your own. Pay little or no attention to how you mix your chemistry.
Don't mark your bottles properly. Fixer before developer will give you pristine clear whites.
Enough for one post. Depressed already. I will rack my brain cells for the next lot. Or someone else may beat me to it. Things we have all done and either regret or laugh at in retrospect...
One more before I post. With older cameras, scratches on the negatives often as not clear up with a small adjustment and/or an isopropyl alcohol clean to the pressure plate. Or there may be a bit of emulsion stuck somewhere behind the plate.
A more common problem with old SLRs is the foam seals have deteriorated from age and these should be replaced. Kits are available cheaply on Ebay but the job is fiddly and tedious. I've done it a few times, never again.