If you often use ssh+tmux combination and ssh keys forwarding, you’ve definitely been in an unpleasant situation:

    connect to some remote machine via ssh and create a tmux session
    use it happily
    detach from tmux and disconnect from server
    connect again (e.g. next day) and attach to the tmux session
    push something to git (or connect to another server)…
    …
    Permission denied (publickey).

Let’s discuss why this is happening.