Even something as simple as an oil change can go wrong. Drain plug broke and you need a new one? Or you forgot to put a gasket and it leaks and now all your oil is on the floor? Well, now you need a trip to the parts store, or order it and wait until parts get there.

In my case, it was a routine maintenance operation: changing brake fluid. I used a power bleeder, and at a third of the pressure specified on the manual, the reservoir ruptured. So now I needed a new reservoir. Parts store didn't have it, and it took 2 weeks to get there.