The word "re-work" sends chills down a mechanic's spine. It means having to repeat a repair because some aspect of the original repair failed. It means an unhappy customer. It means the mechanic may "lose money" because in some cases he won't get paid for having to do the same repair twice. It's a bad deal.
Some re-work is flat-out due to mistakes. We all make mistakes, and they must be corrected. A limited amount of re-work is due to faulty parts. If that's the case, the dealership usually tries to get the manufacturer to stand behind the faulty parts, and to pay for not only the newest set of parts but also the mechanic's labor to replace the faulty parts.
Some re-work just "happens." Some machines refuse to be fixed. All a mechanic can do is tear things apart again, carefully examine the situation, and then determine if there is a microscopic flaw, aberration or misalignment that is causing repeated failure of the piece or component.
Honesty helps. I once repeatedly had to repeatedly unplug the threshing cylinder on a combine. My boss was beginning to wonder if I was misaligning the concave or doing something that made the machine prone to plugging. After the third trip to laboriously dig out material packed between the concave and rasp bars, the operator finally confessed that he had been using the combine to "mow" the grass and weeds in waterways when he crossed them, rather than raising the header over the lush green growth. Once he used the combine only to combine soybeans, his (and my) problems with plugging the concave disappeared.
The most frustrating type of re-work is intermittent electrical problems. Things like seed tube sensors that sometimes work and sometimes don't, or sprayer boom shut-offs that sometimes don't turn on/shut off like they're supposed to. A mechanic can't fix something if it's not broke, and trying to diagnose an intermittent problem is a hair-puller because you can't be certain it's "broke" while your testing, or if the testing has accidentally jiggled a faulty wire or connection and "fixed" the intermittent problem.
That's why, if you call a mechanic to come fix a gushing oil leak, a smoking belt or a smoldering bearing, they usually are confident as they make the repair. It was plainly, obviously "broke." Confidence is harder to come by when the mechanic is chasing some nasty little electrical gremlin causing an intermittent electrical problem.