andy8271 said:
would the money even fit in the wrong slot ? the notes are different sizes ?
ATMs use plastic cassettes to hold the cash. Inside the cassettes are adjustable note guides, so it's not possible to fit a stack of £20s in a correctly adjusted £10 cassette. Unfortunately, not all cassettes are setup correctly.
On the side of each cassette is a holder for up to four magnets. Different denomination cassettes have the magnets in different places. The magnets trip reed switches in the ATM, so it knows what denomination cassette is in each position.
These type of errors occur because the cash is put in the wrong cassette. Sometimes the lids are swapped by mistake. The staff think they are loading a £10 cassette (because that's what the label on the lid says), but it's a £20. In this case, the ATM pays short.
It's also possible for the magnets to be read incorrectly if the cassette isn't fully inserted. This happens on the latest NCR ATMs.
I had this problem myself, years ago. I'd finished repairing an ATM, and got the staff to reload the cash. I asked them to do a test withdrawal, but they couldn't be bothered.
About 15 minutes later, I got a frantic phone call from my office, and went back to the bank. They were extremely upset with me, saying it was my fault. Turns out the staff had balanced the ATM (emptied and counted all the cash), while I was fixing it, and had put it back in the wrong cassettes.
I had to go through all the transactions to prove that it was their fault. Many red faces when I told them what they'd done.
