I used to keep points in stock, rebuild a pump, get it working perfectly and then toss a coin to see if I might be going to get it back in a few days time with a fault again.
We used to replace them with the all electronic pumps which became available but not if the owner wanted to retain the original SU for authenticity.
Dynamo brushes were another favourite - used to keep those in stock as well as the bearings, but I never replaced a Bendix drive as far as I can remember.
But then, if I can remember repairing dynamos I must be starting to lose the old marbles a bit by now.
I think on balance I get fewer problems with customers' cars these days.