The inexpensive retail ATM's we manufacture can track how much cash was dispensed if the dispense was not complete, of if there was an issue and cash was diverted from being dispensed into the reject tray. The terminal will automatically reverse a transaction if it failed to fully or partially dispense the requested cash during a transaction, and will credit the difference back to your account.
I have a hard time believing a machine that costs $1500.00 can do all that yet a $50k ATM on the side of a bank cannot. Banks also reconcile their machines nightly, comparing the cash in the Dispenser cassettes, the reject tray, and the transaction journal reports. It had better all balance. I own and operate several ATMs and I can assure you I check to ensure everything balances..
So if there was One transaction where the machine pulled back the '$300.00 you failed to remove from the machine' and everything doesn't balance, when they reconcile the machine, how hard do you really think it would be for them to figure out ? I say it would take them less than 5 minutes to find where the money went.
If what you mention was actively being exploited the banks would be loosing money and they would not sit idly by, I am positive they would be going after the manufacturer of the machine with a vengeance, and the issue wouldn't be hanging out there for 8 months..