Oo! The percentages thing reminded me of these.
The employee that my friend has working for him at his shop is really, really, really smart. He's an engineering student and I know he's good at math. It absolutely killed me when he attempted to tell my friend that letting people stack their 10% reward discounts (so either giving 30% off or 27% off depending on whether you were adding or multiplying) on the same purchase was the same as giving them the 10% discount on three separate smaller purchases. He had even somehow made a spreadsheet.
I was able to give him a simple-math round-number example to get him straightened out, fortunately, but I couldn't believe he had that kind of basic misunderstanding of how percentages worked. He later told me that he couldn't believe he had it that wrong either. Then again, I did have to explain to my friend that it was better to give a percentage off instead of a flat dollar amount, because giving a flat dollar amount off could easily mean giving things away or at the very least wiping out the sometimes-slim profit margin on a sale, but that giving a percentage off would never wipe out all the profit on a sale.
I also had to explain to my friend that he should be counting the cash drawer after close every night, because otherwise he was going to lose money due to error or theft. It took several incidents where we had to do forensics on the day's receipts after the drawer didn't get counted for a day or two to get him in the habit of actually doing it between every close and open. I don't know if that's necessarily something every adult should know, but any adult who has decided to go into retail sales should know it for sure.