If I execute this command:

wget --no-verbose -o -log -r --mirror --convert-links --page-requisites --include-directories=/assignments --limit-rate=20k http://www.iana.org/assignments/index.html

in an empty directory, the final lines of the file -log are:

FINISHED --2016-10-25 10:27:40--
Total wall clock time: 1h 16m 59s
Downloaded: 7404 files, 71M in 54m 54s (22.0 KB/s)
www.iana.org/assignments/perm-mcast-groupids: Is a directory
www.iana.org/assignments/power-state-sets: Is a directory

The final two lines seem to be incorrect, and likely indicate a logic error.  (The message should also say what wget was trying to do at the time the error was found.)  Indeed, the two items mentioned are directories, but they are directories that wget created, it shouldn't be surprised by them.