ASP.NET Application hosted on IIS7 that is ignoring custom errors and falls back to IIS errors

Question

I have a C# web forms ASP.NET 4.0 web application that uses Routing for URLs for some reason custom errors defined in the system.web section of my web.config is entirely ign

Answer 1

Instead of using:

Response.TrySkipIisCustomErrors = true;

Use this in Web.config: