Anybody who forgets even one detail from his Torah learning because he is negligent in reviewing properly is considered by the Torah as if he is putting his life in danger. Torah protects a person. And if a person retains his Torah knowledge, it serves as a tremendous zechut, a tremendous protection for him. By neglecting to review, to do chazara (review) on his learning, he is putting himself out there. He is putting himself in danger. That shows us the obligation not only to learn Torah, but also to ensure that we remember the Torah. This is not only for our knowledge, but also because, as people who encounter halachic (pertaining to Jewish law) situations, we need to know what to do. Is something permitted? Is something forbidden? We have to have the ability to answer properly.
Therefore, a person who, even though he didn't do it on purpose, was negligent with his chazarot, with his reviews, is considered at fault for not studying correctly. We learn this from the pasuk (verse), "Be careful and guard yourself very greatly. So you don't forget the things that your eyes saw." When we say "things that your eyes saw," it's referring to the Torah learning that was given to a person, and he has to keep it in his eyes—in his mind's eye. A person may think that he's liable even if he forgot some of his learning because it was very difficult to understand, overwhelmed him, and he was unable to remember it correctly. Maybe it was too difficult for him to understand, and that's why he forgot. The Mishnah says, "No, no, no, that's not where we would hold the person accountable." The pasuk tells us, "so that they're not removed from your mind, all the days of your life." We see from this pasuk that when we use the word "removed," we're saying that a person is only guilty if he sits and removes them from his mind, which means he chooses not to do chazarot, not to review. But if he forgets his learning because it was too difficult for him to comprehend and understand, we would not hold this person liable.