Even the best programmers can make the silliest mistakes.

Sometimes you just need a new set of eyes or a good night's sleep to see it. If something's bugging you, stop what you're doing, get up, walk around, and go grab a bottle of water. It can help clear your head, and when you're back, there's a good chance you'll see your problem almost immediately. 3 hours staring at your computer screen, or 10 minutes walking around and drinking some water. The choice is yours.