Sounds like a very insecure, nervous, adhd-type person on his first day at work. At least, I can identify with the guy, it could be a fourteen-year-old me. Just had instructions, never did this kind of work before, a wild customer appears, panic, your 'big, impressive boss' looks over your shoulder, all these new impressions, it would really freak me out. In the end, I would isolate myself from everything and just fall back to memorizing all the steps from the instruction, take bag, open bag, put fruit in bag, take money, give change, and then one step is missing and the plan falls apart. And then the 'boss' takes over and you feel like a complete failure.
Luckily I'm not like that anymore, but I once was exactly like this guy. It doesn't have to do anything with stupidity, maybe after a few customers he got some confidence and did his work just like everybody else.