Even in today's world of automatic debits and swiping money chips lodged in your urethra (I'm beta-testing that right now), people still pay for things in cash. And sometimes that cash comes in large denominations: $20s, $50s, and even $100s. Now, I'm not sure if you know this, but if you're a cashier and your drawer doesn't add up at the end of your shift, some employers take the difference out of your pay. Other employers fire you. That's why accepting a counterfeit $100 in place of a real one would be a bit problematic for cashiers. And beyond that, some businesses have trained their employees to always check for the various official markings of higher-denomination bills, so cashiers do it because, y'know, it's their job.
"Yes, a picture of Nicolas Cage giving Benjamin Franklin a reach-around ... this one's good!"
But don't let that stop you from making a great joke! If you're an exceptionally funny person, when this happens to you, you'll wait until the moment the cashier inspects your bill and say ...
"Don't worry. It's good. I printed it this morning!"
Get it? No, he's not saying he works for the United States Treasury Department; he means it's fake. It's a counterfeit bill. So actually when he's saying "don't worry," if it were true, the cashier should totally be worrying. So it's funny because it's true. Wait, I mean it's funny because it's not true. Wait a minute. It's not in any way funny, and if you do it, I hope that cashier drops the bill in alarm and instantly has security restrain you while she calls the police, advising that she caught the fake-$50 bandit and obtained a full confession.