Counterfeit postal money orders, like counterfeit money, cannot legally be deposited in a bank account.

Bank clerks are not required to absolutely verify the status of a money order before accepting it, and the bank is not required to honor it in any case. The responsibility of a clerk who pays out money erroneously is subject to the policy of the individual bank.
But the bank account holder is ultimately liable for all counterfeit or fake notes fictitious items deposited or cashed through their account.