Our NES needed a new 72 pin connector
Although thoroughly cleaning our new NES the other day was enough to get it booting Super Mario Bros, it wasn't reliable enough. On a bad day it could take five minutes of fiddling with the position of the cartridge and resetting the console to get the game running.
The next thing to try was to completely replace the 72 pin connector. These cost about 150kr, so it's not a huge investment.
After disassembling the NES again, I had high hopes that the old one would look visually older and more worn out than the new one. Something to boost my confidence in the repair before beginning the reassembly. Didn't really happen.
You could really tell the difference when inserting the Super Mario Bros cartridge afterwards though. With the old connector you could practically throw the cartridge in there. With the new one it became quite unforgiving and precise.
That difference in feel when inserting the cartridge meant it wasn't a surprise at all when Super Mario Bros booted on the very first try after finishing the reassembly. It seems to be completely refurbished now!