has it happened before ?
have you sent stuff to this recipient before ? did it go to the correct one as well ?
The most likely explanations are within the contact list
you might have the wrong name for the address or even two entries for a person one with the wrong address or you put something in the cc or bcc box by accident
the first place to look to track it down is to look at the email in your sent folder and see who it was supposed to go to and to click on each of those names to see what actual email address they go to.