Rabbi Shmuel Niman and the Student at the Window
Rabbi Shmuel Niman locked latecomers out of his shiur, until one student climbed the outside wall to knock on his window. He let him in: just this once.
From the article
If there was an issue of students coming late, Rabbi Niman would lock his door at the beginning of shiur and not allow late students in. Rabbi Niman’s shiur room was technically on the second floor of the yeshiva, but the first floor was really just a basement with ground-level windows. One enterprising student was dismayed to find the shiur room door locked. He went outside and climbed up on the outside of the building to knock on Rabbi Niman’s window. Rabbi Niman let him in with a smile, but said, “Just this once!”