What is the most meanest, honest comment you got, that ended up improving you?

We all come across stark honest people, who don't shy away from speaking their mind, even if their words hurt the ones around them. These people can be really a boon when we are looking for real honest opinions and they can be a pain in the rear at sometimes when their raw honesty is not needed.

My question is what kind of such comment made you improve yourself and your methods for the good? Even if what they said hurt you deeply, what response made you realize the truth?