The time I carried counterfeit dollars back to the US

You are all familiar with the changes in US Currency in the past ten years or so.  US paper money now has a new look to deter counterfeiting, and the most obvious change is the size of the picture of the Presidents on the front of the bills.  In Africa, this is called Big Head Money.  They will not accept Little Head Money in Africa - the bills printed more than 10 years ago - so you better make sure you have Big Head Money with you before you go.  At the end of one of our trips in Sierra Leone, I found myself with a lot of extra local currency.  I was with a local friend who has a friend who changes money.  There are no exchange booths at the Sierra Leone airport, and no bank will accept Sierra Leone money back in the US, so all money has to be exchanged in country.  And it is usually with a dude that has a lot of cash on him.  Anyway, my friend called his friend and in a few minutes the money changer arrived.  We negotiated a rate and he started giving me Big Head twenties.  I noticed he also was carrying several Little Head twenties.  I asked him why and he said it was currency that was stuck in the country.  No one would accept it.  The only way it could be of value to him would be to exchange it to an American.  In the US, he correctly said, all money is the same - Big Head or Little Head. Now me being the nice guy, I told him I would take a couple of Little Head twenties to get them off his hands.

Fast forward a week.  I'm home in Houston and filling up my car with gas.  I give the attendant a couple of Big Head twenties and one of the Little Head twenties.  He studies my money for a while and hands the Little Head twenty back.  He won't take it!  "Wait a minute", I said.  "This is America.  They both spend the same here."  He asked me to give it back and he rubbed a pen across it.  It made a dark purple mark on the bill.  He took a Big Head twenty, rubbed the same pen on it, and no mark showed up on the paper.  He shrugged.  I realized I had been passed counterfeit money.

I found out later that not all clerks are as diligent as this gas station attendant, especially when there is a long line at the drive thru and you have put in a long, complicated order......