...or at least it used to. You just finished up that new web application and deployed it to a subdirectory of another .Net app. It worked flawlessly on the dev machine but now you receive the dreaded error message. What happened? Chances are your new app is inheriting configuration setting from the parent app. One fix is to clear or override every offending config setting in the child apps web.config. For instance if your parent app has a setting <pages styleSheetTheme="Red"... but your sub app does not use a styleSheetTheme then you will get an error that the Red theme cannot be found because it has propagated from the parent config. An easy fix is just to specify the non-existent theme in your sub web.config; <pages styleSheetTheme=""... now it's looking for no theme and you have no error.
What if you have 10 or 20 directives that need to be overridden? Wrap the entire section of the config with inheritInChildApplications="false".
<location path="." inheritInChildApplications="false">
<system.web>
<!--anyhing here should not be inherited-->
</system.web>
</location>
Now go buy a new hairbrush with all the money you saved not buying Rogaine :)