I am trying to handle all HTTP errors to my custom error pages, but I find that when there is a % at the end of url, i cannot use config setting or code to handle it, for e
See this 4 part series for configuring custom error pages within IIS: http://www.dotnetscraps.com/dotnetscraps/post/Did-you-know-Enable-Custom-Error-in-IIS-7-75.aspx.
I personally prefer to use Application_Error event to log the errors and redirect user to custom error pages. Note that you need to use integrated pipe-line in IIS to catch all errors otherwise IIS will show its own error page for resources that are not served by ASP.NET.
EDIT: Sorry for the wrong answer. I have XP machine currently that shows 404 for % sign so couldn't verify above. Searching over internet, I found that it's simply not possible to display custom error page for 400 status code. See this question from server fault for more information.
Who said not possible?
Response.TrySkipIisCustomErrors = true;
OR
<configuration>
<system.webServer>
<httpErrors existingResponse="PassThrough" />
</system.webServer>
</configuration>
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