[Long Jon No Silver]

I read a story once where the character went back in time and taught the Romans modern medicine or gave them modern medicine. Something like that. The outcome was the Earth became so overpopulated thousands of years later, that it was like a writhing ball of humans. The aghast time traveller went back to ancient Rome again but this time he left a rifle.

[who_would_fardels_bear]

If the stories of how the conquistadors inadvertently decimated Native American populations via diseases are true, then a modern human going back in time could be very problematic.

You most likely would have to wear thick robes, keep at a distance from others, and act rather mysterious in order to discourage people from coming near.

Oh wait, that's pretty much Merlin's modus operandi. So...