In Thunderbird, because it sometimes stupidly does this thing where when I forward emails, the To or CC/BCC boxes just can't be selected at all. (I think it's an extension causing it but it didn't used to, only now that TB's updated more recently...)
It was an important domain registration email with password and logins in it!
I'm only asking because I'm paranoid (but also interested) - it didn't actually go to anyone did it? like the original sender or the email maybe? What happens when this happens - I haven't received any 'undelivered' mail server message back yet.