I had an ATM eat a check I tried to deposit once. I contacted the bank and they had me give them the ATM location and date and approximate time of the transaction, along with what I thought should have been the result (debiting N dollars to my account). They debited my account immediately and then investigated. A week or two later I got a letter saying that they'd found the check and the debit would remain on my account.

My guess is that when problems do occur the banks are, generally, very responsive about resolving them because a loss of confidence in ATMs would be rather disastrous for retail banking in general.