I read in the Liverpool Echo some years ago about a bloke who was prosecuted after taking advantage of a company’s mistakes.
He bought a large TV from Costco which was displayed as being (say) £1,999. At the checkout, it rang through as £199. He didn’t query it, the till operator didn’t query it, he walked away with a nice cheap large TV. And at that point hadn’t done anything particularly wrong.
But when he went back a second time, and a third time, and a fourth time when the mistake still hadn’t been fixed, he was knowingly taking advantage of their mistake to create a situation of obtaining property by deception (aka theft). He was convicted.
The first instance was considered to be the store’s own risk – contract law says that the price on the shelf is only an “invitation to treat” so isn’t binding, the contract is made when the till operator effectively says “I offer to sell this to you for the price I’m ringing it through at” and you accept that offer. If the till operator (or the company) has made a mistake in the price that they offer, that won’t necessarily invalidate the contract (though it can do – the doctrine of mistake comes into play). But from that criminal case it seems that if you induce the mistake from the company with intent to take advantage of it, that can be considered to be a criminal offence.
So I’d agree with thecaptain here – banking the second cheque (30 years ago!) I think would technically be theft; it’s an intentional act of taking advantage of a mistake where at least clarifying the position was not difficult to do.
But I digress…