Just had that happen.
It's not the end of the world - no harm done and the forwarding was completely innocent but... It just really makes you stop and think... ANY emails you send out... you've just got to be careful about what you say! Because, you never really know...
When you say "the world", do you mean a wider audience than you intended or the world world?
Has not happened to me, to my knowledge. But... I try to take the view/follow the guideline that if I did not want to see it on the front page of a newspaper, I do not put it in writing - email, letter, internet forums (!).
There have been some well reported instances here in the UK of people sending intimate emails to 'a friend' and that friend sends it on to a handful of friends and before half a day is out, that email has gone around the world several times and of course, made their way into the newspapers. These emails were invariably sent from work and various people in the above matter have either been disciplined or lost their jobs... something about bring their employer into disrepute.
Even with using personal/internet email, 'a friend' could still send it on to a handful etc etc. The advantage of using personal/internet email is that if you are using an alias recognised by your 'friend', it does not necessarily mean that the rest of the world will know you by that alias.
The other thing I think people have not quite woken up to is that with the proliferation of camera phones and the digital era of being able to send that photo to a website or even upload onto YouTube in seconds, is that you have to work harder to keep your personal life just that, no matter how 'anonymous' you think you are.