IIS 7.5 returns error 401 instead of 500

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http://serverfault.com – I have a strange problem with ASP.NET MVC web application. IIS 7.5 throws back error 401 instead of 500 when application error occurs. This happens in production environment only, dev or staging works fine; the difficulty here is that I don't have access to the production environment so I have to replicate issue on my own to see what is wrong. Any suggestions? (HowTos)