Witnesses taught that religion was a "snare and a racket" until 1950, when they decided to become a religion themselves.
They taught that God would make black people white in the New World.
They criticize the Catholic Church for teaching until 1873 that the "curse of Ham" resulted in the black race. The WTS does not admit that it taught the same thing well into the 20th century.
They taught that God lives in the Pleiades.
In 1960 they suggested that God would use "cosmic rays" to drive his enemies insane, and antimatter to destroy the world at Armageddon. In 1971 they proposed he would use antimatter to dispose of the billions of corpses after Armageddon. Now they teach that the lucky survivors will get to bury them.
EDIT: Almost forgot, one of the best ones: The Witnesses introduced their own calendar so they wouldn't have to use a calendar with pagan names for months and days. It never caught on, so the Witnesses still use our pagan calendar and apparently are no longer bothered by it.