If you pride yourself on being competent at finding information, please skip this post. This post is for people who know they screw up sometimes. Oh and by the way, competency can be overrated!
Has this happened to you? You do a search on the internet, and find exactly what you're looking for - but forget to copy the URL, or bookmark it or add it to del.icio.us. Later you try to refind the information, and can't do so even if your life depended on it. It's one of my more irritating nightmares, not being able to find something which I know exists, which I had found once before.
I think I know what's going on here. The knowledge of the answer is interfering with my ability to ask a good question. The difficulty is that sometimes it works - there are times when knowing a bit of the answer helps you find the complete answer. But sometimes it just doesn't work - why? The same reason why false memories can be created, the same reason that identification evidence from eye-witnesses is extremely unreliable. I think what happens is that our memory of the answer is slightly false. The difference might be tiny, only a matter of a few words or letters, but if you're doing searching for answers, even that could a problem.
The solution - other than trying to take good notes and making use of your browser's history feature - is to forget the answer. Return to a beginner's mind on this topic. If you're successful, you will most likely unconsciously do the same search which you did when you first found this information. But doing this is easier said than done.