Paraphrased from the words of Jeph Jacques of Questionable Content:
The sharpest lightbulb in the box... being the broken one of course.
A seemingly flattering compliment that is meant to be explained to the victim after they thank you.
A: You know what? You are the sharpest bulb in the box.
B: Awww, thanks that means a lot, :)
A: The sharpest one being the broken one of course.