I've noticed that people often manage to become moderately good at something, but seem to reach some sort of peak at that level and then rapidly start getting worse again.
I've seen this happen with videogames (most notably Starcraft 2), pool games, badminton, and even drawing/painting. People can get from complete beginner to a mediocre level in all these things, and then at some point they will start getting worse at them again. Not all the way back to a beginner level, but far enough to notice a serious difference in results.
If there are no physical changes that are causing this, is there any explanation for this problem with learning?