Backport:

    or a message. Apache will sometimes offer additional information

    regarding the problem/error.</p>

    <p>URLs can begin with a slash (/) for local URLs, or be a full

    URL which the client can resolve. Alternatively, a message can

    be provided to be displayed by the browser. Examples:</p>

    <p>URLs can begin with a slash (/) for local web-paths (relative

    to the <directive module="core">DocumentRoot</directive>), or be a

    full URL which the client can resolve. Alternatively, a message

    can be provided to be displayed by the browser. Examples:</p>

    <example>

      ErrorDocument 500 http://foo.example.com/cgi-bin/tester<br />

    href="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q294807"

    >Q294807</a>.</p>

    <p>Although most error messages can be overriden, there are certain

    circumstances where the internal messages are used regardless of the

    setting of <directive module="core">ErrorDocument</directive>.  In

    particular, if a malformed request is detected, normal request processing

    will be immediately halted and the internal error message returned.

    This is necessary to guard against security problems caused by

    bad requests.</p>

    <p>Prior to version 2.0, messages were indicated by prefixing

    them with a single unmatched double quote character.</p>

</usage>