Your timing leaves something to be desired.

[RealUnimportant]

When I take payment, I have very specific routines. Once you've let go of your money, I keep it to one side, fish out your change & keep it in my hand while your tickets print, then place the ticket & change into the return mechanism. Then I hold onto the money you used to pay until you walk away, so that it's there for comparisons in case of any disputes - this is a time-perfected technique based on experience.

So today, when you turned straight back & said "I gave you a tenner", you were just a smidge too quick, because the fiver you'd actually given me was still in my hand, see? ... Yeah, I don't blame you for not trying to press the issue. Buh-bye!

[PepperElf]

how much do you want to bet some of those "they short changed me!" stories at PFB were more like this one... where they paid with a 5 and then claimed they gave a 10?

[Gaki]

These get really great at the gas station because they'll give me a ten and try to tell me they gave me a twenty. Well Sir, if you HAD given me a twenty, I'd have one in my drawer...

[RealUnimportant]

...I'd have one in my drawer...

One of my first "my change is wrong" on the railways involved coins, but was in this vein; I was accused of giving someone 1p coins instead of £1 coins. Unfortunately, no fare on the railways requires the use of pennies so we don't keep any on hand... Which meant there's no way I could have given any as change! Unsurprisingly they didn't ask for a count-down, and my total was accurate at the end of my shift.

[sms001]

I noticed a guy accusing one of my c-store workmates of shorting him once. We didn't run the drawer and I don't know if it was short. Second time, a month or two later, the drawer DID come up short after he pulled it on a second co-worker. When I saw him again after another lengthy interval I made sure to wait on him. Whole transaction goes fine - one item, a five tendered. He turns to go and (ala OP) turns and says the bill was a twenty. I pick up the bill from my till ledge and snap it and say "Nope. Five spot. Nice try."
Wrong thing to say. Turns out he's a personal friend of the owner. I told him that the guy was already in my radar from previous occurrences and I was just firing a shot across his bow to keep repeats to a minimum. Boss wasn't having it, and I got a dressing down for embarrassing a friend and a customer.
I thought (and still do) a lot of this boss, but really. When does someone who is stealing from you stop being a friend?

[Merriweather]

I noticed a guy accusing one of my c-store workmates of shorting him once. We didn't run the drawer and I don't know if it was short. Second time, a month or two later, the drawer DID come up short after he pulled it on a second co-worker. When I saw him again after another lengthy interval I made sure to wait on him. Whole transaction goes fine - one item, a five tendered. He turns to go and (ala OP) turns and says the bill was a twenty. I pick up the bill from my till ledge and snap it and say "Nope. Five spot. Nice try."
Wrong thing to say. Turns out he's a personal friend of the owner. I told him that the guy was already in my radar from previous occurrences and I was just firing a shot across his bow to keep repeats to a minimum. Boss wasn't having it, and I got a dressing down for embarrassing a friend and a customer.
I thought (and still do) a lot of this boss, but really. When does someone who is stealing from you stop being a friend?

If the same guy has done it before, what the heck are you supposed to think? Giving someone the benefit of the doubt the first time (as in, treating them like they're mistaken, not scamming, & providing proof, not as in handing them the extra money, of course) make sense, mistakes happen. But not usually repeatedly to the same person.

Just a few days ago I was at a fast food drive thru, I had a $10 & a $20, and was thinking I should use the $20 as I needed smaller bills for something. when the cashier handed me change for a $10, I kinda stared for a minute, fumbled to see my purse, with a "I thought I gave you a.....nevermind, my mistake" as I saw the $20 still in my purse. I did give her a "sorry about that" and a smile. But I'm sure she thought she was about to go against a scammer.

[Ashaela]

I actually never had someone pull this on me while I was checking, but I had an interesting occurrence at a clothing store when the clerk gave me back too much change. I looked at it, said "Actually, I gave you fifty," and she panicked trying to figure out how much she'd shorted me. I felt really bad that she must have had an SC throw a fit earlier or she wouldn't have reacted that way.