Here’s a quick gotcha that I just remembered. If you are customizing a Publishing Site and you have just created a new masterpage, you may run into a situation where anonymous users are getting a 404 error trying to view the website. This may be obvious to some people but it took me a good couple hours to figure out that in a Publishing Site, approvals are turned on by default. The user you created the masterpage with will be able to see it but until you publish and approve it, no anonymous user will be able to see the masterpage (since it would be a violation of the approval process). I’m thinking Microsoft could have come up with a better error message in this scenario, but I guess 404 makes sense because to that user the page really doesn’t exist yet.