If your a vim user and use both PC's and Mac's then you have probably command+w in the Terminal app before when trying to change vim windows. Luckily for me I just have to reattach to my tmux session to but its still annoying, so here is how to override the short cut so you never have that problem again.
Here is a very good article: http://tech.inhelsinki.nl/gnu_developement_under_mac_os_x/