protocol version mismatch (client 8, server 6) when trying to upgrade

Pretty awesome hack, if you need your tmux working and not want to lose all your sessions:

$ tmux attach
protocol version mismatch (client 7, server 6)

$ pgrep tmux
3429
$ /proc/3429/exe attach

original post on Google Plus - https://plus.google.com/110139418387705691470/posts/BebrBSXMkBp

This basically tells you, that you already have an (old) tmux-server running and the new tmux can't connect to it because they don't understand each other anymore. Exit all your existing tmux sessions and start a fresh one using the new version and everything should be fine.

This perhaps isn't ideal for you, but I created a chroot, and ran tmux from inside it:

sudo -i
debootstrap stable /chroot
mount -o bind /dev /chroot/dev
mount -o bind /dev/pts /chroot/dev/pts
mount -o bind /sys /chroot/sys
mount -o bind /proc /chroot/proc
chroot /chroot
apt-get install tmux ssh
tmux
ssh user@localhost
apt-get bla bla bla

This ensures that the tmux environment doesn't change during the upgrade