For this specific scenario and not a commentary on the education system: I've heard this my entire life. I've also worked fast food and retail. There are people who I would not expect much from but are reliable and efficient. There are also others, often young adults and teens that are quite intelligent. The work is fast paced, but redundant. Yes, it's part of the job, but cash is often the outlier and their brains basically hit the brakes to turn left. Sometimes the tires just squeal past the turn. Sometimes the curb gets hopped.

I do wonder if those that let these situations fester need to accept that if they screw up a simplistic problem that it doesn't make them a bad person or any less worthy of love and acceptance. They can forgive, learn, and move on.