If you've tried to perfect a difficult, long task by repeating it multiple times, you'll know what I'm talking about.
You start tackling the first few phases of the problem, until you succeed. Then you fail later on and you must start over. After enough restarts, you'll eventually start having trouble performing the first few phases you thought you had down pat, way before those where you were really trying hard to get through. It's as frustrating as short-lived however: after a few more tries, you'll return to previous or higher levels of performance.
If you are the visual kind of person, here's a terribly confusing and misleading graph about what I'm talking about: