Just replace
bind -n C-M-w send-keys M-w\; \ run-shell "tmux save-buffer - | xclip -i -selection clipboard"\; \ set -g prefix 'C-b
with
bind -n C-M-w send-keys M-w\; \ run-shell "tmux save-buffer - | xclip -i -selection clipboard >> /dev/null "\; \ set -g prefix 'C-b
After playing around this issue Thanks to Severyn Kozak who suggest using /dev/null
I think the problem in some commands that don't return output or exit status like xclip so redirecting the output to /dev/null will be enough for copying from paste buffer to clipboard.
I couldn't get why you use Send keys and set prefix commands ,but for tmux 1.8+ you should use copy-pipe command instead to copy selected text to paste buffer and clipboard
Copy and paste method using emacs-mode from this answer:
bind-key -n -t emacs-copy M-w copy-pipe "xclip -i -sel p -f | xclip -i -sel c " bind-key -n C-y run "xclip -o | tmux load-buffer - ; tmux paste-buffer"
Using vi mode "prefix C-[ to enter copy mode >> v to highlight text >> y to copy text to both paste buffer and clipboard >> prefix p to paste ":
set -g mode-keys vi bind -t vi-copy 'v' begin-selection bind -t vi-copy 'y' copy-pipe "xclip -i -sel clip" bind p paste-buffer