My first job was as a cashier at a large-chain grocery store.  I knew I probably wasn't going to make it long on that one when I accidentally insulted someone.
A very well to do asian looking family comes in.  I check through about 150 dollars worth of groceries.  They pull out a credit card (looking item), and start trying to put it through.  I rung it up as a credit card purchase.  Several times it didn't go through until I took the card and tried to put it through on my end. 
I mention that the credit card isn't going through and asked if they had another one.  The wife looked at me and half yelled, "They're food stamps, handsomehunk!"
I never put it together.  They were wearing REALLY nice clothes, shoes, accessories, the works...  It looked exactly like a credit card, and no body had ever explained to me that Food Stamps were not really stamps anymore.
Back then I had a horrendous habit of blurting out stuff before thinking.  I said, "Those things are real?"  The Manager had come over at that point, and sorted it all out.  But she was a coward and didn't fire me on the spot.  After a weak of about half my usual hours at the ABSOLUTE worst times, I took the hint and left on my own.