How can one HVAC technician make so many mistakes?

He looked at me oddly, and I told him he didn’t give me the bill

I wasn’t sure if the HVAC technician was really an HVAC technician. He was making so many mistakes that I was afraid he was going to break the furnace more than it was already broken. I talked to him and started asking some questions about the furnace and how long he had been an HVAC technician. He asked if it showed? I wasn’t sure what he was talking about until he said he had only gotten off his apprenticeship and I was the first repair he was making without his mentor. I knew everyone had to go out on their own at some time, but I couldn’t believe he was this nervous. He said he loved being an HVAC technician, but it was much easier when he had someone there to talk to. While I was talking to him, he went through the inspection process, and figured out what he was doing wrong. It didn’t take long for him to figure out what he had done wrong, and how to repair it. After another fifteen minutes, he was done with the furnace and getting ready to leave. I stopped him before he walked out there and told him he forgot something. He looked at me oddly, and I told him he didn’t give me the bill. He laughed and said he didn’t need to take care of that when his mentor was with him. That was one mistake that was justified, and I let him get away with it. He explained what he did to the furnace, and talked me through the bill, but he wasn’t able to take my credit card without calling the HVAC company.