I just typed multipass shell and accidentally hit Return before typing my instance name in, and was very surprised to see multipass launch a new VM. This was particularly confusing given that the short help says "Open a shell on a running instance" (emphasis mine, of course), so that's what I assumed it did.

Once I'd seen this happen, I looked at the full help text, and I think we also have some confusing wording there. Specifically, "If the instance is not running, an attempt is made to start it" is inaccurate, as multipass did not attempt to start it, it instead launched a completely new VM.