The British East India Company — which ruled over 200 million people — was brought down partly by a grease rumour about animal fat on rifle cartridges
In 1857 the British introduced the Pattern 1853 Enfield Rifle to their Indian army. Loading it required biting the end off a greased paper cartridge.
The grease was rumoured to be cow and pig fat — sacred to Hindus, forbidden to Muslims.
For Hindu Sepoys the cow was sacred. For Muslim Sepoys the pig was haram. Biting that cartridge wasn't a military drill — it was a forced violation of both religions simultaneously.
The British denied it. Nobody believed them.
The resulting rebellion killed tens of thousands, lasted 18 months, and ended with the complete dissolution of the East India Company after 200 years of rule.
One of history's most consequential rumours — and it was about rifle maintenance grease.
Sources: Saul David — The Indian Mutiny 1857 / Kim Wagner — The Great Fear of 1857 / Parliamentary Papers 1857-58