Sadly, I can top this. At a Sprouts recently, a manager was training an apparent new hire. When a third and fourth customer approached, she opened the next register to help them. Their registers are in pairs, so she was literally right next to the newbie, but facing someone else. The person in front of me paid cash and the newb punched in the amount of payment. Then he says "It says eighty. How do I do that?" Instinctively, I answered, while pointing to his cash drawer, "It's three of those and one of those." The manager smiled and thanked me, as did the newb.
I've worked retail for so many years that it took me a second to realize that he literally couldn't recognize the value of the specific coins, much less do the addition. Facepalm.
Another time, I bought about six bucks worth of stuff at Trader Joe's and paid with a ten. The clerk pulled about seven ones out and started to hand them to me. "Uh, that's gonna be to much" says I. "Thanks for being so honest" says he, "I'm really bad at math." [In my head, I say] Um, no. That's not math. The register did that for you. That's just counting. You really work as a cashier at TJ's and you cannot count to four? I thought that one took the cake until the very recent Sprouts experience.