The wget manual states: ‘-N’ (for timestamp-checking) is not supported in combination with ‘-O’: since file is always newly created, it will always have a very new timestamp. A warning will be issued if this combination is used.
(In this section of the manual): http://www.gnu.org/software/wget/manual/html_node/Download-Options.html#Download-Options The warning is printed if you do: wget -N -O http://example.com But the warning isn't printed if you do something like: wget -N -O- http://example.com Maybe the statement in the manual should be revised to avoid confusion? See: https://github.com/docker/docker/pull/12857
