What happens if I mistype a recipient’s bank account number when transferring money online?

It depends where in the world you are doing this. The transaction most likely would eventually be rejected. It might be stuck for a day or two, but since the benefactor name would not match the Bank Account Number / Branch Number, the transaction would bounce back.

The caveat being, that in your part of the world, the Bank Account Holder's name is matched with the Account number.