Have you ever tried deploying a sub website on an hosted environment? and your getting a "reference not found" build error? i.e. you have your root domain has http://www.samcalma.net/ and you want to deploy another domain http://www.samcalma.net/designpatterns/ . But everything is not working as expected, the web config on the root web site is taking precedence of whatever entries you don't have on the second level web application.
This is basically explained on this blog entry: ASP .NET Web.Config Inheritance
The "remove" directive does the trick.