In modern versions of tmux, ESC doesn't quit copy mode like it did in GNU Screen.
CTRL+C exits copy mode, but CTRL+C in normal mode sends the deadly signal SIGINT. We don't want users to get into the habit of using such a dangerous key to exit copy mode. So we should teach them a better way.
Each time a user enters copy mode, please show them a message that teaches them the safe way to quit: "Copy mode (press <F1> for help or q to quit)."
<F1> help actually doesn't exist yet. Please see ticket [#48]: "If I press F1 in copy mode, show me a list of keybindings".