The great sage Rabbi (Yehudah ha-Nasi, the editor of the Mishnah, who lived circa 135-217 CE) sent one of his disciples, Levi ben Sisi, to the town of Simonias in the Galilee to serve as its teacher and judge. The townspeople were so excited to have a student of Rabbi himself that they organized a welcome procession and lined up their theological questions in advance.
Levi arrived. They asked their questions. He could not answer. Not a single one.
Some of the questions were simple matters of halakhah that any yeshiva student could have handled. Others were well-known disputes in the Mishnah. Levi stood there blank. The knowledge he had carried in his head for twenty years had vanished.
Word got back to Rabbi Yehudah. He was puzzled. He had personally trained Levi, and knew his student's mind well. Finally the diagnosis emerged. Levi had grown proud on the walk from Rabbi Yehudah's academy to Simonias. The honors planned for him, the welcome committee, the title of chief scholar of the town, had swelled his head. And pride, in the rabbinic understanding of the mind, evicts knowledge from the places where it lives. Torah refuses to stay in a puffed-up brain.
The townspeople were unimpressed. Rabbi Yehudah intervened, rebuked Levi, and humbled him. Through that humiliation Levi's mind cleared. The knowledge returned. When he was small again, he could once more recall what he had learned.
This story from Yerushalmi Yevamot 12:7, preserved in The Exempla of the Rabbis (Gaster, 1924), is the rabbinic diagnosis of a familiar modern ailment. The moment you think you have arrived, you cannot remember how you got there.