I had a very nasty experience with a non-written down password which I forgot while on holiday.
In theory, it should have been recoverable. Two words, which I remembered, on the memory palace principle, but separated by a number, generated from two other numbers, picked from a set of a few easily memorable numbers which I could remember. What I could not remember was exactly which two, and what mathematical process I had applied to them.
I spent the evening with a calculator trying to get into my computer and failing again and again.
So I took it to the local computer shop who did work for us at school back in the day. He has a number of programs which he has used to get into people's computers. I told him the words and the numbers, and a week later he had totally failed to get in.
Fortunately, before going away, I had copied all the files off the thing so it was empty of data, so completely restoring it to factory setting and rebuilding all the software wasn't too much of a problem.
I was quite pleased at having an unbreakable password.

When he and I had done this, guess what? The number presented itself to my mind. I had always felt it wasn't the best number because the maths had produced a number in which two digit patterns were repeated to make the four.
I now have a little list.