Even though I was taught about sunk costs in business school twenty-five years ago, I have had to learn this lesson the hard way. Most of the time that we make a follow-on investment defensively, to protect the capital we have already invested, that follow-on investment is marginal or outright bad. I have seen this again and again. And so we try really hard to look at every investment based on the return on the new money and not include the capital we have already invested in the decision.
via Fred Wilson- A VC