Currently, when your render finishes, the render dialog has 2 buttons. The 'Save' button, and the 'Render' button.
I had just finished redering a scene wich took 7 hours to render overnight, and I wanted to save it. Accidentally, I hit the button next to it, without warnig, it just started rendering again. so now I have to wait another 7 hours before I can save the image. I think that is a waste of time!
Please let there be an "Are you sure you want to render this again" dialog in the future, or another solution, because currently, I'm not as happy as I want to be...