Salut polonus,
It's like when you take a test. You, generally think you have everything down and expect to get a high grade. However, when you do get your paper back, you see it is lower than you expected. You look at one of the questions and say "How could I've got that one wrong?"
Well, I think that the human can't focus so much on a certain test, and if the attempt is too much, losing control over your instincts is common, thus you might make the mistake or two (even more) during a test.
OR after the test you understand the subject a little more.
This is common to me when I take French class.
I used du instead of de, le instead of la, and so forth. But you learn and move on, not making those mistakes again.
Au revoir,
~Donovan