Test the program
The purpose of testing a program is to confirm that the behavior of the program meets the customer's expectations
If it doesn't, you must explain how and why it fails to meet those expectations.
(Sometimes the customer's expectations prove to be unrealistic relative to the circumstances in which the program is used.)
If the program fails to meet the customer's expectations, you may be in for a long and unpleasant negotiation.
It will behoove you to manage the customer's expectation back at the first stage of the process.