A very handy tip from Wilfred Hughes:
Emacs tip: If your instance hangs and won't respond to C-g, you can use `pkill -SIGUSR2 emacs` to force emacs to stop whatever it's doing.
— Wilfred Hughes (@_wilfredh) October 28, 2015
Emacs 24.5, on OS X at least, sometimes hangs for no apparent reason. I’ve used this tip several times to get things unhung.
UPDATE: I forgot to add that on OS X you have to either use the -i option or specify Emacs (note capital letter) as the process name.