>Hi,
>Should we tell the truth of our program bugs to our client? Or, we make a story/other excuses ?

- your client is an individual person or a person with limited or no reporting on the IT issues within one's organization.

The answer is a loud YES. It's a question of trust.

- your client is an organization or a person reporting on the IT issues within one's organization.

Then the answer is not that straighforward.

You should try to say what they (not he or she) would like to hear on the subject.

A lot of organizations (especially big ones) have a strong aversion to risk.
A bug is a risk situation.

Be sure to evaluation the damage associated to announcing a bug before doing it. A person within the client organization (the one that signed the first check, the project manager...) may suffer from the bug announcement.
In this case, you'd better be creative.

François