What happens then?
The ATM sucks the cash back in and recredits the customer's account.
Well that's how most banks treat it, except NatWest and RBS which don't; instead, up until now, they have been crediting a "dump account".
However, that seems set to change as Finextra reports that Royal Bank of Scotland and NatWest have set aside a £10M reserve to refund up to 300,000 customers who made a withdrawal at the ATM but walked away without the cash.
The bank is apparently checking its records over the past seven years and will repay the money, plus the interest earned on the sum, to customers who forgot their cash.
This brings a whole new meaning the phrase "bank error in your favour".