Cotação de documentação do Apache :
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.
Portanto, se você quiser redirecionar o erro 400, você deve remover o comentário da diretiva ErrorDocument
no arquivo de configuração do Apache httpd.conf
da seguinte forma:
# 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"
e reinicie o servidor Apache.