Replay Attack
From CryptoWiki
Basics
"A Replay Attack is an exploit that can occur when two forked crypto-currencies allow transactions to be valid across both chains.
A Replay Attack does not mean someone else has control over your funds. A Replay Attack can only copy an existing transaction from the new forked blockchain and make an identical one in the old blockchain (or the other way around).
Keep in mind that transactions sent without proper replay protection can be subject to a replay attack months or even years later. This is why not sending or exchanging an asset forked without Replay Protection is the safest way to go if you want to be able to claim your forked coins down the line.
You can read more about replay attacks in this article."
