If for some reason you thought it's a good idea to update tmux to a new version without closing your current tmux sessions first, you might see this error when trying to attach to those sessions after update:
$ tmux attach
protocol version mismatch (client 7, server 6)
To attach to your sessions, use this awesome hack (credit: this stackexchange answer):
$ pgrep tmux
3429
$ /proc/3429/exe attach