Prior to the planning of the client's website, the client may have had only a passing interest in web site design, layout and features. Now, with their own site under construction they become very interested, analyzing a lot of other sites and in the process generating some great ideas for extra features and pages. This can derail the process of building a specific website to a carefully prepared timeframe and budget. Sometimes it is easy to accomodate the changes, but where these are major diversions from the plan it is best to simply document them, complete site as planned, and then undertake the changes as a second phase or upgrade to the site. This usually adds very little cost to the overall project.
Late information is another issue to address.
If some text or pictures for example will not be ready when the project commences this should be discussed early. We can always work around missing components and complete other pages providing we know up front and have a clear idea of when the material will be available.