There have been some issues recently with the num package which, as of OCaml 4.06.0, now lives on as an independent library. In order to maintain as much compatibility as possible, the num package installs its files to OCaml’s lib directory, rather than to site-lib/num or $OPAMROOT/$OPAMSWITCH/lib/num.
This was a problem for opam system switches, since opam shouldn’t write things outside $OPAMROOT.
I’ve written a more complete explanation of the problem on my blog - Homebrew users in particular may like to clean up their /usr/local/lib/ocaml directory now, rather than waiting for a new version of ocamlfind to cause num-related pain.