A common problem is not understanding which objects are available in a given context. For example, you might attempt to set properties for a Session object in a client script, or attempt to use the Internet Explorer object model in a script running on a different browser.
Make sure that the objects you reference are available in the current scope. Be aware that objects (such as ASP built-in objects) are not an inherent part of a language such as VBScript; instead, they are part of the environment in which a script is running.