Have you ever been testing new web applications in SharePoint and constantly gotten the Runtime Error (Server Error in '/'). If you do get this error the real error might be displayed in the event log like the one I found below.
Exception information:
Exception type: HttpException
Exception message: The current identity (NT AUTHORITY\NETWORK SERVICE) does not have write access to 'C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files'.
Request information:
Request URL: http://sharepoint.devcow.com/
Request path: /
User host address: 127.0.0.1
User:
Is authenticated: False
Authentication Type:
Thread account name: NT AUTHORITY\NETWORK SERVICE
A common problem that I just recently saw when creating site collections using the Network Services account for the app pool was a restriction to the ASP.NET Temporary files. This is due to the fact that the Network Services account is a restricted account, so by default it will not have permissions to everything another account, say an administrator, would have.
Keep this in mind next time you create a new site collection or web application and get the Runtime Error screen. Go and check the event logs to see if the error is in there before you try to perform the actions asked on the screen, like changing the web.config which won't work.