Counterfeit?

So Barb and I were at Zaphod last night for a few drinks. Barb went to the washroom and left me $6 to buy another drink. I promptly approached the bar to purchase a Stella Artois. After I settled back into my seat the server walked over to me with the $5 bill I had handed her and said "This is counterfeit you know."

To which I replied, "Are you kidding?" As I figured Barb got that very five from the bar. She then showed me the five under the black lighting of the bar, as if my innate currency knowledge would then show that it was indeed counterfeit. I was still quite doubtful but I paid for my drink with another bill and went back to wait for Barb. She couldn't remember where she got the five - from Zaphod or a cafe. So I couldn't yell at them for giving us counterfeit money and then refusing to take it.

The five was reexamined this morning in comparison to another I had at home. Honestly I can find no difference with the exception that the "fake" one is more worn.

Barb will go to the bank today to find out the truth. Here's my question: Who counterfeits a $5 bill? I mean is it even profitable to counterfeit a $20? It's got to cost almost $5 to make a bill that realistic. And how do you launder a $5? Go buy a chocolate bar and make off with your $3.93 in booty? Seems highly suspicious to me.