In September, someone posted on reddit about not being able to get his Tmux configuration work when SSHing into Ubuntu 10.04 from a Mac running iTerm2.
The source of the problem is iTerm2 sending
\e[1;9D for ⌥ + arrow keys. I asked @gnachman why iTerm2 sends
\e[1;9D and not
\e[1;3D and he answered 9 was inherited from the old iTerm (see the conversation on Twitter)…