A sin is never a mistake. Use of the word "mistake" is modernism.
To commit a mortal sin, one must recognize with any doubt that he is committing a mortal sin, and also realize that that particular sin is recognized by the Church as a serious sin. Then, he must deliberately choose to do what is morally wrong. In other words, when a person deliberately chooses to commit a mortal sin rather than to obey God's laws, such a sin is never a mistake.
Isn't this the sin of Satan when he stated, "I will not serve." Self-will and pride are involved in any deliberate serious sin as pride is the root of all evil.