at the end of the day, whether its the bank overpaying you or you are being billed for electricity actually used but not paid, the civil law principle of unjust enrichment will apply

art 2142 of the civil code states:

"Certain lawful, voluntary and unilateral acts give rise to the juridical relation of quasi-contract to the end that no one shall be unjustly enriched or benefited at the expense of another. (n)"

this principle was used by the courts in the vibram case decided by the supreme court

of course, there are certain exceptions to the above rule and that is where legal training comes in and the worth of your legal counsel will be determined