A professional should be able to give you an estimate of his time pretty accurately based on the job to be done. Usually when out of the ordinary is more than 15% it would be discussed at the time it was discovered.

I'm not sure about that your line means "If they guy is "eating" his time for mistakes he's entitled to the whole amount of time he spent." If he's 'eating' time for his mistakes I'd like to know the total hours and then how many hours he discounted from his total time.

Putting a gasket in backwards isn't something that should happen. Dropping parts and breaking them shouldn't happen. A good professional generally doesn't do that.

I've made mistakes and ended up working for free because of it. Do that a few times and you become more attentive and careful on what you're doing.