No one, for example, takes Gibbon's argument on the Fall of the Roman Empire seriously anymore; similarly, no one takes the argument that Islamic cultures economically failed (in comparison with Europe) because of anti-capitalist religious precepts seriously either. Yet both were a part of serious teaching a few decades ago (the age of some textbooks).
I remember one textbook I had as a child argued that the reason that Lowland Scots prospered in comparison with Highland Scots was due the Protestant work ethic bestowed upon them through Prebyterianism - in comparison, the Highlanders succumbed to their lethargic Catholic proclivities. Hilarious in hindsight, but slightly disturbing as real teaching.
"Now this is a totally brain damaged algorithm. Gag me with a smurfette." -- P. Buhr, Computer Science 354