I've been freelancing for quite a few years and I'd say this is an expected problem. Not that I wish it to happen but I won't be surprised if it does. That's why it is very important to define the scope as clear as possible in the agreement.
But by the time this problem arises it's usually too late. So I ask myself: are subtasks A, B, and C expected from a reasonable person's view?
Without getting into how a reasonable man is constructed in court cases, think of this example: if I order a burger with soda, is it reasonable for the restaurant to charge me $1 for the paper cup, $0.5 for the wrappers of the burger? Are customers expected to bring their own cups? If the answer is yes (everybody in this town brings their own cup), then the charge is reasonable and you cannot argue with it.
A more realistic example: if a client asks for a login page, then it make sense that there should be some mechanism for users to signup. If the developer's answer is "you only asked for a login page, you have to manually write SQLs to signup users" then I'd say this is unacceptable. But if the client says "users should be verified by sending SMS to their phones", then I'd say "no, this is an extra feature and the cost is $x".