We all know someone that's intelligent, but who occasionally defends obviously bad ideas. Why does this happen?

Decius wrote:
This is not a bad set of observations, but it avoids the fallacy I think is most common.

You're emotionally invested in a particular outcome, and you see the idea at hand as being related to that outcome, and so you are unwilling to sacrifice it because you feel that sacrificing it means sacrificing your ultimate objective.

This is one of Powell's Rules:

Avoid having your ego so close to your position that when your position falls, your ego goes with it.

which has been memed at

http://www.memestreams.net/thread/bid9831/