Quotation from Apache documentation:
Although most error messages can be overriden, there are certain circumstances where the internal messages are used regardless of the setting of ErrorDocument. 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.
So, if you want redirect error 400 you should uncomment
ErrorDocument directive in
httpd.conf Apache configuration file as follows:
# Some examples:
#ErrorDocument 500 http://foo.example.com/cgi-bin/tester
ErrorDocument 400 /cgi-bin/bad_request.pl
#ErrorDocument 401 /subscription_info.html
#ErrorDocument 403 "Sorry can't allow you access today"
and restart Apache server.