If you SSH into a remote system from within Emacs, you can have problems with the EDITOR environment variable on the remote system. This can arise when you invoke an application that calls the editor specified in EDITOR.
It turns out there’s an easy solution: the with-editor package can take care of this for you. If you’re using magit, you already have with-editor installed. If you’re not using magit, you can install with-editor from Melpa. Better yet, just install magit, you won’t be sorry. In any event, the details are in this very useful Emacs Stack Exchange entry.
If you’re a Tramp user, you really should take a look at the entry.