ASP.Net. Why am I not seeing my defaultRedirect page even when customErrors mode="On"

Here's the situation: -

I have

    <customErrors mode="On" defaultRedirect="erro开发者_JAVA百科r.html" />

in my web.config.

I also have custom error handling code in Application_Error (in global.asax.cs) that checks for certain well known exception types and redirects to custom error pages for each. If the exception is not a well known type then I simple return from Application_Error without calling Server.ClearError() so the default custom handling should take care of showing the page error.html.

But whenever my app throws up an unhandled (and not well known) exception I end up at the default Asp.Net error page - you know the one I mean, the yellow screen of death...

Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed. 

Details: To enable the details of this specific error message to be viewable on the local server machine, please create a  tag within a "web.config" configuration file located in the root directory of the current web application. This  tag should then have its "mode" attribute set to "RemoteOnly". To enable the details to be viewable on remote machines, please set "mode" to "Off".

I have checked the obvious stuff (like double checking that I am not throwing another error in Application_Error and I am not accidently' calling ClearError. It is definitely something fishy with this one web app (I made a little test web app and the custom error handling worked as expected) - that should rule out oddities in my machine.config file.

I am going to keep on scratching my head and trying things but in the meantime I thought I would put the question out here to see if anyone had any bright ideas.

PS. I am working on Windows 7 Business (IIS 7.5) but the same behavior is observed on Win server 2003 (IIS6).

Thanks Joe and Click for having a look but this problem is all my fault!

One crucial bit of information that I omitted from the description is that my web page flushes the response before the request is finished (see * below if you want to know why I do this nastiness).

It seems that once some content has been sent to the client, the customError handling no longer works. I can't say that this is a failing of the custom error handling because it would probably take some dark voodoo to tell a browser to redirect to once it had started rendering content!

Having said that I was hopeful that adding redirectMode="ResponseRewrite" to the customError node would help (maybe get the error page below the content I have already sent to the browser?) but that fails too. Nevermind.

A quick google yields this http://forums.iis.net/p/993221/1313916.aspx - and a chap on there has a work around that I will try. The technique suggested in http://forums.iis.net/p/993221/1399583.aspx#1399583 works. It is far from ideal (we have to read the web config) but it works.

(*) why am I flushing the response before the request is finished? Some of our pages return large reports which can take several seconds to generate. We send a "PLease wait + spinny gif" message to the clients at the start of these requests and flush that so that the browser renders the message + icon whilst it waits for the rest of the response. I think I shall investigate a less "hacky" way of achieving this.