As always, I am not good at linguistics but as far as I know there are children's mistakes in learning their mother languge and they learn gradually their own language by making mistakes. However; second language learning process is not unlikely first language learning in its trail-and-error nature.
Competence is "what you know" and Performance is "what you do". Here if you made mistakes you would refer to your performance because of some accidental things including, your fatigue, doing sth hastily, and so on. On the other hand, since you considered sth right, in fact, that was really wrong, it would be errors based on your competence.
I don't know whether I explained the above passage correctly or not!