The Karaite Schism
Anan ben David rejects the Oral Torah and the authority of the sages; Karaism arises, a movement that split away whole communities for centuries.
Around 4527 a rupture occurred in Baghdad whose consequences lasted a thousand years. Anan ben David, of a distinguished family, was a candidate for exilarch — head of Babylonian Jewry — and was passed over by the sages in favour of his younger brother. Anan declared that he accepted only the written text of the Torah, and that everything the sages had added was human invention. His followers were first called Ananites, later Karaites, 'people of the Scripture'.
In principle it sounded like a return to purity. In practice the Torah proved unreadable without the Oral Law. The Torah never defines 'work' on Shabbat, so the Karaites banned any fire left from Friday and spent the day in darkness and cold. Taking 'frontlets between your eyes' literally, they abolished tefillin. Without the rules of the calendar they returned to sighting the moon and the ripening barley, and their festivals drifted away from everyone else's. Their marriage prohibitions grew so wide that finding a spouse within the group became nearly impossible.
Karaite communities arose in Jerusalem, Egypt, Persia, later in Crimea and Lithuania. The decisive answer was not force but a book: Rav Saadia Gaon met them with systematic argument and won back the intellectual ground.