2016/10/14 追記

直ったっぽいので下記手順は古いと思います。

$ brew info tmux
tmux: stable 2.3 (bottled), HEAD
Terminal multiplexer
https://tmux.github.io/
/usr/local/Cellar/tmux/2.2 (9 files, 639.8K)
  Poured from bottle on 2016-09-30 at 12:01:37
/usr/local/Cellar/tmux/2.3 (9 files, 650.5K)
  Poured from bottle on 2016-10-03 at 10:12:30
/usr/local/Cellar/tmux/2.3_1 (10 files, 652.4K) *
  Poured from bottle on 2016-10-13 at 22:59:50
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/tmux.rb
==> Dependencies
Build: pkg-config ✔
Required: libevent ✔, utf8proc ✔
==> Caveats
Example configuration has been installed to:
  /usr/local/opt/tmux/share/tmux

Bash completion has been installed to:
  /usr/local/etc/bash_completion.d

tmux使おうとするとエラーが

[warn] kq_init: detected broken kqueue; not using.: File exists

無視して使えたけど気持ち悪い

ググったら解決方法あった

下記コマンド実行でなおった

$ brew uninstall --force tmux
$ brew install --HEAD tmux