Not necessarily. Time travel. We could exist in the same time currently, albeit lightyears apart, but you simply jumped backwards to visit the Greeks. And that seems the most likely, unless freak wormholes really are a common occorunce. Like I said, the Greeks had nowhere near the required technology to make the leap. You at least have space exploration.

Repercussions? Imagine sustained communication with a vastly less developed world. No legal systems in place to deal with allocation of resources. Military capabilities nowhere near equal. And considering your advanced technological capabilities, they probably worshipped you as gods, or at the very least sorcerers... Need I go on?