A student once struggled with an assignment and learned to improve through practice. When another student makes a similar mistake, the first student is better placed to help rather than humiliate.
Forbearance is not mere silence. Sometimes a wrong must be corrected, but the timing, tone, and aim should support reform.
عَثْرَة means a stumble or lapse. It does not mean a person should deliberately make mistakes or commit sins in order to become patient; the lesson concerns learning wisely from experience.