Don't Forget Your Learning!
The mishna says that if someone forgets his learning he is חייב מיתה, it then qualifies that by saying it only applies if a person actively tries to forget what he learned.
According to the Abarbanel it is a continuation of the previous mishna where it says that stopping to admire nature while learning is a serious transgression (because it causes one to forget one's learning). Here too it refers to someone who wasn't fully focused on their learning, causing them to forget their learning.
Rabbeinu Yonah says the crime is not a lack of focus while learning but not reviewing his learning. "שגגת תלמוד זדון". The Bartenura says that it's not a punishment for forgetting, it's a result of losing the protection you get from the Torah you learn.
Delivered at the OU Israel Center