In OISF LibHTP before 0.5.31, as used in Suricata 4.1.4 and other products, an HTTP protocol parsing error causes the http_header signature to not alert on a response with a single \r\n ending.
https://redmine.openinfosecfoundation.org/issues/2969
https://github.com/OISF/libhtp/pull/213