By default, browsers hide JavaScript error messages.
Please enable web console. Error messages will be displayed there.
How to enable:
   In firefox menu, "Tools"-->"Web Developer"-->"Web Console"

Click the button "works". JavaScript

        <script>alert('Button [Works] clicked');</script>

will be executed.

Click the button "Error". JavaScript

        <script>myFunc(); alert('Button [Error] clicked');</script>

should be executed. But the popup doesn't appear.