Retarded Cashiers

So I was buying food at my college campus' cafeteria and the bill came out to $8.50 (or something like that) and I handed the cashier a $20 bill and she gave me back $1.50. I told her that I gave her a $20 and she insisted I gave her a $10. Then, she proceeded to show me the receipt that she printed and pointed and said, "See, it says $10... change $1.50." And I said, "Yeah, thats because you put in the register that I gave you $10." Fucking genius. I didn't have the energy to argue with her. But cashiers out there - please mind your q's and numbers. Do you think she's smart enough to do that with everyone and pocket the dough? Cuz if thats the case, I totally give her credit. But if she's just stupid - then that's retarded.