Windows 2003 License

I am a system engineer. I am writing to ask you to give some direction.

We are working for our client as a system part.
My client is using Windows 2003 std server with AD environment.

They have 27 users but at the moment they have just 19 UserCAL. So many time we can see error log, which is out of license.

Of course, we are going to purchase more license as many as we need.

From here, I have a question:

Is it possible that users who have no license on the server might have problem to access server resources such as third party applications (MySQL etc)?

Thanks.