It is remarkably easy to send an email to someone. It is also remarkably easy to send an email to the wrong someone. I stopped using the nickname feature of my mailer once after accidentally sending email to my co-workers that was intended for my parents: it was just too easy to make mistakes like that. This is a very strange new kind of miscommunication. You can screw up six times before lunch with this new breed of technology. Wrong numbers tend to sort themselves out pretty quickly; you get stopped before you say too much of anything, and it takes a certain rare kind of cluelessness to leave a rambling multi-minute message on the wrong answering machine. It's now possible to effortlessly send a completely coherent message that is nonetheless nearly completely incomprehensible to its actual recipient.
This is actually what stuns me most about ill-routed email: the bizzare interpretive situations it spawns for the person whose inbox the letter winds up in. I got mail from a fellow including his brother's resume for our software lab technician position. Funny, I thought, don't we already have one of those? And hasn't he been here longer than I have? And why me? I'm a leaf node on the org chart. The mail was supposed to go to an address differing from my own by one letter, but it took me a goodly long while and some back-and-forth mail to figure this out. The email sort of made sense, and I struggled way to hard to make full sense out of it.
Something similar happens when you hit "reply" instead of "forward" and you meant to tell your friends what an idiot the person you just replied to is. And yet, if you wrote, say, "Can you believe this shit?" the mistake may not even be apparent to the would-be cussee, and you may receive a confiding, "I know, isn't this whole situation just insane?" reply from said idiot. That wasn't the message, no, not at all in the way it was meant, but the human mind will go to great lengths to try and puzzle out something that looks like it makes sense. The pure authoritative condifence of an email sucks you in, makes you think your own memory must be in error, makes you search in vain for the connection.