If you work retail yo most likely have a counterfeit detector pen lying around the register. This trick prays on people's insecurity about money so only do this to regular customers with a good sense of humor, I cannot be held responsible for you getting you butt handed to you. The way this works is when you mark a counterfeit bill with the pen the mark turns black otherwise it will be light brown or yellow. You have to make the mark come out black.
*Edit* This may or may not work outside the US. I'm am not certain on how to detect counterfeits elsewhere but the concept is simple and could be molded to your area most likely.
Step 1: Material Needed
Aquire a counterfeit detector pen, a black sharpie (the closer resembling to the pen the better) and choose your victim. I have done this with everything from a $1 bill to a $100 bill and people will believe it, just find someone with a good sense of humor like I said before.
Step 2: What to Do
When the customer purchases their items grab the bill and look at it inquisitively like you suspect something's amiss. Grab your sharpie, being sure to conceal the pen as well as possible so they can't see it's a sharpie, and put a small black line on the bill approx 1 inch long.
At this point the customer will ask why you just checked it or something along those lines. State why it's a counterfeit and say you can;t accept it. Trust me on this when I say this is the hardest step, You have to keep a straight face. The customer will be able to see it in your face or the way you speak and know right away that it's a prank.
Step 3: How to Close the Deal
If they don't believe you take your real counterfeit detector pen and show them the directions on it. Among tests I found going along with the prank for about a minute is the best time limit to put on this prank so be smooth and fast when you do it. After they're either becoming too frustrated or start to reach in their wallet for a new bill let them off the hook, The results are normally worth it and you keep the regular customer around longer showing you have a sense of humor.
Step 4: Additional Tips
Along with the previous indicated warnings I'm going to provide another, don't do this if you have a line in the store as it will just slow people down in checkout and everyone will just be frustrated with you after you reveal the prank. Do not abuse this prank either, only do it 2-4 times on April Fools day and maybe, Maybe, 1 more time throughout the year otherwise it just won't look real and be too practiced. In 3 years at 7-Eleven I've only done this 4 times as to keep it more or less secret, if I did more then word would probly have gotten around and I'm sure this Instructable won't help too much.
Another good way to do this is if you have a co-worker with you don't tell them about the prank and lead them on pretty much the same way as the customer so you have 2 people thinking it's a fake. They'll probably check it themselves and be confused when it comes out as real so you may have to end the prank early but it's even more worth it then.