A user of the nsopenssl module was having problems getting the module to listen on the right port. He was sure it was nsopenssl's problem. I sent him an 'instrumented' copy of the module so he could see what ports the module was binding to etc.
He came back saying he felt rather foolish because it was a problem in his configuration. It was something rather obvious, to which I responded:
If I could count the number of times my blind assumptions about where a problem lay have bit me in the butt, I'd have been without a butt for a long time now.
What I have learned is that the simplest thing is (almost) always the answer. When I start repeating to myself, "But it has to work this way" then I know I'm in trouble and do some backtracking. The Universe, it appears, is logical. But we aren't.
The amount of time he spent on this problem speaks to our ability to force ourselves onto a particular path even though it was clear to him that the problem was obvious, but he simply couldn't change his context to see it.
Most things are pretty obvious, but we have problems with them, I think, because we make them more complex in our minds than they really are.