You would be surprised. I've run several mailing lists for both technical and non-technical people.
Always, without fail, you will get someone that signs up, confirms and then mashes the spam button two emails later while sending you irate "HOW DID YOU GET MY EMAIL ADDRESS UNSUBSCRIBE ME OHGODWHATHAVEYOUDONE?!" emails.
Well yes, forgetting that you signed up is still possible if there's a confirm step. Being signed up (maliciously or accidentally) by someone else, however, is the situation I was referring to and confirm steps exist specifically to prevent that.
Not possible if you have a confirm step. And you should have a confirm step.