I wrote before Christmas about overactive Bank of Ireland ATMs. Exactly the same issue has just arisen in Australia:

Consumer groups are calling on the Commonwealth Bank to explain what went wrong with its ATMs on Tuesday, with a glitch resulting in some customers withdrawing excessive amounts from the machines. Police charged two men in Sydney on Wednesday with fraud for allegedly withdrawing extra money from the faulty machines during the meltdown.

As with the Bank of Ireland ATMs, the CBA machines operated in standby mode, but if you take money to which you are not entitled, it may very well constitute theft – and even if it doesn’t, it will certainly give rise to contractual or restitutionary duties to return the overpayments. I have this news via Legal Eagle on Skepticlawyer, who argues that

… the Commonwealth Bank will definitely have recourse to recover the money from individuals who have taken advantage of the glitch in its computer processes, subject to the defence of good faith change of position. It’s my theory that one of the reasons behind the rise of unjust enrichment law is electronic banking and the mistakes which arise therein. Seriously! I’ve spoken before about the paradigm case, Chase Manhattan Bank N.A. v Israel British Bank (London) Ltd [1981] Ch 105 … There are also some Australian cases, including ANZ v Westpac (1988) 164 CLR 662 … If one took money from an ATM and did not realise that it was paid by mistake, there is a defence of good faith change of position (David Securities). … Let it be a lesson to others out there — if an ATM is malfunctioning, don’t simply take the money and tell all your friends — tell a bank employee.