You've probably read my many exhortations to get requirements documented in writing as part of your contract, and design your prototype and your acceptance tests to those requirements. If you've not done that, you, and not your customer, are at fault.

When he says, "That's not what I wanted!" you simply show him that, at the time the contract was let, it WAS what he said he wanted. When he says it's not, you tell him, "Bring money ... Lots of it!"

RE