Custom error message displyed only while debugging

I have this entry in my web config.

<customErrors mode="On" defaultRedirect="~/ErrorPage.aspx" />
<compilation debug="false" targetFramework="4.0">.

Basically I am trying to show custom error page whenever there is an unhandled exception in the application. I get the custom error page only when I am debugging the application i.e. when w3wp(worker) process is attached. It shows the generic error page when not debugging.

I am obviously missing something here?

via Chebli Mohamed