If Bitcoin experiences a fork, the biggest risk faced by an average user Is a replay attack.
What is it?
When Bitcoin split into different versions, such as BTC1, 2 or 3 etc, each Bitcoin holder will retain his holdings and receives an equal amount of the forked cryptocurrency.
The address, private key and algorithm is the same on every chain. The transaction format is the same too, meaning that a transaction which is broadcasted on one chain can be relayed it to a node on another chain and receive validation.
This is “Replay Attack”.
Simply put, when you transfer BTC1, Your BTC2 or BTC3 can be fraudulently transferred away too.
Now, many forks added replay protection to mitigate the risk of replay attacks.