Quote from: Jet Cash on May 24, 2018, 12:15:25 PM

Quote from: jseverson on May 24, 2018, 10:11:11 AM

The complications start when you start claiming them. Once a fork is launched, you're likely to find a lot of wallets, in which you will need to import your Bitcoin private key to gain a corresponding amount of the forked coin. Fake wallets are going to take your private key, and steal any Bitcoins within. Make sure you only use the official wallet for the fork and empty the Bitcoin address whose private key you will import beforehand.

This is extremely important. The danger comes from a thief replaying the transaction you made on the new chain. If he replays it on the original Bitcoin chain, then he can claim the coins in the transaction from your Bitcoin wallet. If the wallet is empty, then he can't do this.

Most of the answers in this thread should have been really helpfull for the OP, i didn't see many mistakes being made by other posters. I did, however, wanted to elaborate on this statement.
If, for example, bitcoin forked into replayattackbitcoin (a fork without replay protection) and you have 1 unspent output funding address 1MyoriGinalAddress before the fork, what could happen if you spent this unspent output on the replayattackbitcoin network?
1) You create a transaction using the unspent output to fund 1AddressGeneratedbyAseller
2) you broadcast the transaction on replayattackbitcoin's network
3) the attacker saves this transaction and broadcasts it on bitcoin's network
4) the transaction gets included into a block on both the original and the new network's blockchain

The end result is that address 1AddressGeneratedbyAseller will become funded on BOTH chains. The attacker cannot use your transaction funding 1AddressGeneratedbyAseller to fund a thirth address owned by the attacker.
In other words, the attacker has to convice you to fund his own address on chain A, then replay the transaction on chain B, otherwise he'll gain nothing from the attack (he can still perform the attack to annoy you, but he won't have any financial motivation to do so).

I actually made this diagram a long time ago (copyright: me). It shows a replay attack and a simple way to protect yourself from such an attack.