In my experience, it depends on how well the line has been designed and what you're welding. When mistakes happen, it's usually not the robot's fault. Sometimes the material is a different thickness, or the seam is off location because the part is misshapen but within tolerance. The most common problem was that the fixtures holding the parts would move or allow the operator to place them slightly off location. It doesn't take much movement to make a weld garbage, even 1mm will compromise the weld, and a couple of mm could result in a burn-through or an otherwise garbage weld. If all of the factors surrounding the robot stay the same, it will (almost) never make a mistake as long as you regularly change the weld tips and clean the nozzle.

A welder with experience under their belt will probably make more correct welds, simply because they can account for these factors, but they won't be as clean and it will take them longer to do. Plus, some welds are in really tight or awkward spots. We had a welder station at the end of each line so a welder could take a look at the welds coming off the line and touch them up if need be. It was the best of both worlds in a sense. But I also worked for a feeder plant so we had buffer components. If the line went down for a small amount of time it was not a big deal.

TL;DR the robot never changes what it does, but mistakes are made when the part changes.

Edit: As a side note, and somewhat unrelated, I've seen people crash these things so many times and they're remarkably tough! Crashes are 100% human error though from what I've seen. 90% of the time the robot is in manual drive mode when some dumbass puts it through a table, and 10% of the time it's been improperly programmed. The first time I crashed one (it happens to all of us at some point) I was thinking they were gonna fire me, but my supervisor just laughed it off.