Custom Error Page not showing

Question

For some reason, when I get an ASP.NET runtime error, it's not loading my custom error page

<customErrors mode="On" defaultRedirect="app_offline.htm" redirectMode="ResponseRewrite">
    <error statusCode="404" redirect="app_offline.htm"/>
        <error statusCode="500" redirect="app_offline.htm"/>
</customErrors>

That's in my web.config.

I'm still getting this though and it's not loading my error .htm page:

Server Error in '/' Application.
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 remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.

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

<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="Off"/>
    </system.web>
</configuration>


Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.

<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>

Answer 1

I'm pretty sure that app_offline.htm is a reserved filename in ASP.NET that will always be served if present on the server.

Try renaming it to error.htm and updating your <customErrors /> block to match.