The following example is perhaps not the most important, but it is certainly a fascinating look at how a false belief can snowball and significantly impact cultural practices. Our example is the humble fork.
The early Christians endowed the Devil of scripture with the symbols of Pluto, the Roman ruler of the Underworld. In late Hellenistic and Roman iconography, Pluto sported a “bident,” a two-pronged weapon similar to Poseidon’s trident. (This was perhaps inspired by, or maybe reflected in, Seneca’s Hercules Furens, in which Pluto uses a trident to drive Hercules from the Underworld [558ff.]). A Byzantine scholiast writing on Euripides’ Phoenician Women suggests that bidents and tridents had become interchangeable in Late Antiquity. By the Middle Ages, Pluto’s bident/trident had become the property of Satan, who succeeded him as imaginary ruler of the Underworld (Hades), which, as demonstrated by Dante, had become identified with the Christian Hell.
Thus, when the Byzantine princess Maria Argyropoulina, niece of Emperor Basil II, arrived in Venice in 1004 to marry the son of the Doge and brought with her a case of golden two-pronged forks, which had been in use in Constantinople for more than three centuries, the Venetians threw a fit. The local clergy condemned the fork as decadent and as diabolical—the very instrument of the Devil. When Maria died two years later of plague, the Venetian clergy proclaimed it God’s judgment on the infernal fork. St. Peter Damian (in his Institutio monialis) preached that a Venetian princess (probably Maria, but perhaps another—she is not named by Peter) died a miserable death because she used “a certain gold prong [a fork] wherewith she actually conveys her food to her mouth, instead of using the fingers God gave her for that purpose.” For the next four or five centuries, no one in Western Europe would be caught dead wielding the devil’s utensil. In 1573 the Holy Inquisition investigated whether Paolo Veronese was utilizing infernal symbolism by providing Jesus and the disciples with forks in his Last Supper, which he was forced to rename Feast in the House of Levi to keep his forks.
(Medieval texts and woodcuts indicate that forks were in use elsewhere, but they seem to have been used only in private to avoid condemnation from the Church.)
“God protect me from forks!” exclaimed Martin Luther in 1518 (or 1515), according to tradition. (I can't verify the source for this statement.) Protestant clerics would preach against diabolical forks down to the end of the Thirty Years’ War. Before they rode brooms, European witches were assumed to ride on the large fire-forks used in early baking.
Catherine d’Medici eventually used a fork, and was promptly condemned as a wicked witch of a foreign interloper in France. Eventually, Charles I of England declared it a useful device and acceptance soon followed. But even then adoption was not universal. In England, for example, the fork did not catch on until the eighteenth century, and even then more conservative types continued to denounce it as a miniature of the Devil’s own utensil, or at the very least, an ungodly luxury. Eventually, a compromise was found: Forks would be made with four prongs, not three or two, to avoid any connection to the Devil’s device.
By the 1850s, the fork had become a standard eating utensil, thanks largely to aristocratic and royal patronage, so much so that by 1850 The Living Age could confidently write that to “eat like a Christian” was to eat with a fork, in contradistinction to the practice of the uncivilized heathens of the East, who still “lift their food with their fingers.” (Imagine what the Victorians would have made of McDonalds!)
While all these examples had additional political and social factors beyond simply the fork, the entire process of adopting the fork, an instrument now considered a given part of the dinner service, was delayed by 800 years largely because early Christians decided the “true” nature of the pagan gods was that they were demons and devils, and therefore formed a prejudice against the fork due to its chance resemblance to the pagan/diabolical bident.
Had this prejudice not existed, Maria and Catherine would have been (and were) condemned on other grounds, but the fact remains: an “alternative” theory about the true nature of the ancient gods impacted European culture for eight centuries. We still have four pronged forks because of this “alternative” theory. What effects would accidentally arise should we, with equally poor evidence, proclaim the gods were actually Atlanteans or aliens?
Post-script: Wouldn't it figure? Someone went and wrote a whole book about this last month, which of course I only discovered after researching and writing my post. It would have saved me some work.