Woolly mammoths walked the Earth while the pyramids were being built (well, one small corner of it).

Oxford university was teaching before the Aztec Empire began.

I think I read through an entire AskReddit thread of these. A few I came across on my own and was surprised by: Fibbonacci of the numbers fame could have met Ghengis Khan, Benjamin Franklin could have talked to Isaac Newton, and Galileo was literally the same age as Shakespeare.