I believe that most people are to some degree familiar with this story. One of the most immediate causes for the Indian Rebellion of 1857 was the rumor that paper cartridges supplied to native soldiers (sepoys) by the East Indian Company were pre-greased with pork and beef tallow. Such tallow would made them forbidden to be bitten by Muslim and Hindu soldiers respectively. This is a fact. It is also a fact that paper cartridges were in fact greased with a number of substances and that proper manual-of-arms of the time included biting the paper cartridge open in order to allow the gunpowder to ignite inside the gun's barrel.
But was the rumor itself true? Were cartridges really greased with tallow from pork and beef?
Good question - to which there is at best a murky answer, which is: quite possibly, but – incredible as it may seem, when the consequences of the rumour were so serious – despite a full enquiry into the antecedents of the rebellion of 1857, we still don't actually know.
There will always be more to say. But, while you are waiting for fresh answers to your query, you might like to check out the full response I wrote up to your question when it came up here a few months ago.