When I run terminal-notifier from tmux on OS X 10.10 Yosemite, it hangs:

terminal-notifier -message "his name was robert paulson"
# ... hanging

It works fine when running it from outside tmux. This may have something to do with reattach-to-user-namespace. My tmux is configured with this line:

set-option -g default-command "reattach-to-user-namespace -l zsh"

Which was taken from the instructions from this page.