I've been thinking a fair bit these last few months about the notion of misaligned incentives. Both professionally and in the public policy sphere, people optimise for what's best for them individually; if you want a particular outcome, you need to make sure that everyone involved has an incentive towards making that outcome a reality.
I recently came across this perfect expression of the idea, which I pass along here without further comment:
It's true: never let the guy with the broom decide how many elephants can be in the parade.
[Merlin Mann said that.]