I've done the occasional favour, for part costs. Mates son, new BSO, but left it in the garden. 6 months later, a wreck. Changed all the cables and get it going again. Another mate, had a decent GT MTB - 00's machine with the rotating BB shell (part of the rear suspension) and brimming with XTR - did a general service but the brakes were dead (perished bladder etc) - as he'd picked the bike up cheap, he didn't want to spend the £100 on new Deore brakes. Ended up being left in my garage for months - had to remind him to collect it too many times. Never ridden after that. I've also fixed up the neighbour's bikes. Fortunately, nothing in the last few years.
I used to be called on as the family IT expert all the time - SIL used to bring the PC round regularly as it had been broken again by her son (virus etc). Eventually said it's not worth my time reformatting and rebuilding the machine every time.