Debugging offers a most excellent opportunity to practice science - Observe, Hypothesize, Test, Repeat. And just like regular science, bugs span the entire range of trivial to the head-explodingly-elusive.

Observation is in particular quite a hard practice. On several occasions I've realized that the evidence was staring at me in the face from day one, but my eyes were too clouded to see it. Sometimes, it helps if you know the system inside-out, but at other times that itself happens to be a blinding disadvantage.