I have had to go out and fix a second leak two or three times over the past two decades. I can clearly recall two particular times when it was a minor leak in the same area I had been working, so I assumed that the work I did on leak #1 caused leak #2. I mean that the pushing and pulling and so forth may have causes a weak joint to let go nearby. You can't prove or disprove that type of thing. Both leaks may have been leaking when I started, so then it was 'my fault' for not spotting the second leak.
Either way, I get out there fast, offer the explanation that the second leak may have been caused by my work, take care of the thing, and then figure out the money last.
On both of the jobs I have in mind, we found some middle ground for the followup repair. I offered to pay for it all, but both time the customer offered to help - for example we were working on a 20 year-old tub and they were comfortable covering the extra cost of materials or simply offering a generous tip once the thng is done.
Those two jobs I have in mind both happened many years ago. Nowadays, I will put the spa back on the ground and run it with the repair site open for a couple of weeks, looking for more leaks and letting any remaining water work out of the wood and any remaining wet foam that I may not have gotten to. The most recent one was on a used spa which we simply brought into our warehouse.
After the leak was found and fixed - a Classic which had a ton of foam removed from the bottom - we set it on a pallet for the water test. The customer came to visit every couple of days, fascinated with the 'guts' of his tub. When we were all happy that it was staying dry, we flipped it, foamed it, wrapped it and delivered it.
Since I have started doing this 'open wet test' I have yet to have a callback on a leak repair.
One other thing: leaks in the motor compartment often come in pairs: fix the shaft seal on the motor and then that part drys up nicely, only to reveal that the ozone tubing had a slow drip all along....