Theoretical that could happen...

You run a bitcoin node and have Bitcoin. Your last transaction was over 2016 blocks ago. You have friends that regularly transact more often, some in the past day (144 blocks for simplicity).

Unbeknownst to all, a separate chain has been being mined in secret based on 1000 blocks ago (about a week). It becomes the active chain having the most work performed. Your funds predate this and are entirely safe. Some of your friends find that coins they had received on the previously valid chain have been returned to the sender thus lowering their balance.

As unlikely as this seems, it is possible and you should think through how youd react as well as how to push out the inevitable noise

Do you now wait for 1000 confirmations before considering safely received?

As a node runner do you reject the new tip? Do you keep tabs on bitcoin-cli getchaintips

As a miner, do you pull out of mining pools or create new ones? Do you wonder if some of the hash of the pool was being redirected at the other chain? Do you use stratumv2?

If 1000 blocks is too many what if it was 5?