stop apache injecting it's own 404 not found page to the custom 404 error page

I'm having a problem implementing custom 404 error pages. my setup is a front-end apache 2.2.6 proxy request other than static file to the backend app server(apache 1.3.36)

i send 404 header back along with error page when user request a non-existing page to the backend server. I do get the error page back but somehow the backend apache server injec开发者_如何学Gots its own 404 not found page in the error page as well.

here is what injected to the not found page by apache:

Not Found
The requested URL /product/8jd4776 was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/1.3.36 Server at www.example.com Port 8888 

any idea why this is happening?

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set "ProxyErrorOverride On" with ErrorDocument in the front-end apache server solve the problem.

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The content length of your custom 404 message must be 512 bytes or more.