Thanks for the response. If I remove

redirectMode="ResponseRewrite"

, it still does not redirect to ErrorMessage.htm, which is a simple htm file that can't throw an asp.net server side error. When I view source after requesting PageThatThrowsApplicationException.aspx, the html contains

<form name="form1" method="post" action="PageThatThrowsApplicationException.aspx" id="form1">

which tells me the server responded with the html of the page that threw the exception rather than the html of ErrorMessage.htm.

I'm using IIS6 so

Response.TrySkipIisCustomErrors

is not available. There is no master page or global.asax error handling.