I've been gradually bringing my old rigid Dawes MTB up to reasonable standard. It's a hack, so doesn't have serious money spent on it.
The other week I replaced the rear cantis with a pair I found lurking in a box, and replaced the front brake lever with one that works with the V-brake that a previous owner must have fitted.
Then the chain started jumping. So...find a block - I had to buy that; replace the chain - from another box. I greased and adjusted the rear hub as well - impressive how an old, cheap Shimano hub runs as smoothly as new.
Took it for a spin, and the middle chainring was skipping. On inspection the chainset - a nasty rivetted budget thing - was totally ker-nackered. Ordered a cheap one from Parkers; had to wait as it was out of stock. I finally got to fit it yesterday. A few more teeth and the rings sitting further in meant the old front changer wouldn't work. Another root in the boxes brought an old deep-cage triple changer to light, so fitted that. It needed a longer cable, so I searched out a new one. I then had to adjust the routing so the cable would pull the right amount.
Eventually I managed to get out for a few hilly miles. Everything worked.
It's also had a new headset this year, so I don't think there's much else to go wrong.