Every once in a while you hear stories about people getting hit with ridiculously high charges, such as for cell phone or cloud computing, e.g.:
In the cell phone case, this can happen when (for example) a customer roams internationally and doesn't realize that they had agreed to a contract with absurd prices (like $15/MB).
In the cloud computing case, this can happen when (for example) a customer doesn't properly shut down a machine.
Sometimes, this is a result of the vendor making it difficult to keep the bill in check, such as by designing their website in such a way that you need to check a non-obvious page for billing.
Other times, this is just a result of the customer simply being caught off-guard.
Companies often forgive these bills for (what I assume are usually) PR- or cost-related reasons, but I've been wondering:
What legal basis (if any) might the billed party have for challenging such cases?
For the sake of this question, please assume:
The bill is correct per the contract.
The charges were not caused by malicious entities (e.g., DDoS attacks, hackers, etc.) or force-majeure events.
A reasonable person would agree that such a customer would have taken steps to avoid such charges if they had realized the consequences beforehand.
The vendor can reasonably forgive most of bill if they wish to. (e.g., perhaps their actual incurred costs were much lower.)
The question is not related to healthcare.