It's not possible to correct.  Client will receive a check if bank account isn't active.

It's Sunday today - go to church and pray for a rejection.

An Efile error?  meaning it wont let you Efile until you fix a number?

Or your error, meaning you transmitted a return with the incorrect bank account number on it?

It is a good idea to ask the client for a  check marked void, to use to enter the bank information and then triple check it for accuracy before e-filing. Just my opinion.

You only triple check it?  You must run really fast with those super sharp scissors 😉