The wget(1) man page says under the "-nc" option:

           When running Wget with -N, with or without -r or -p, the
           decision as to whether or not to download a newer copy of a
           file depends on the local and remote timestamp and size of
           the file. -nc may not be specified at the same time as -N.

But under the "-N" option is nothing:

       -N
       --timestamping
           Turn on time-stamping.

This might be confusing because users reading the documentation might expect "-N" to be documented under "-N" rather than "-nc". Presumably it should say something like

        -N
        --timestamping

           Turn on timestamping. When running Wget with -N, with or
           without -r or -p, the decision as to whether or not to
           download a newer copy of a file depends on the local and
           remote timestamp and size of the file. -nc may not be
           specified at the same time as -N.

        -nc
            ...
            -nc may not be specified at the same time as -N (--timestamping).