Not sure if this is a bug but this what I did.
Create and send email - in outbox folder. Did not refresh.
Decided I wanted to edit the message. Went to outbox, opened message, started editing then got distracted so left open with changes.
As sync is done every 10 minutes, sync came along and message was sent (with changes) even though I had not clicked on Send.
I do think whilst a message is open it should not be sent. Of course, I should not get distracted but that is a different problem.