If you can mine a block with one of your transactions in it and keep it in stealth, there is an opportunity for you to double spend the money. If a merchant accepts the unconfirmed transaction, you can transfer him this earlier transacted currency. Next you publish the earlier mined block, which was kept in stealth, before your new transaction is confirmed on network.
https://academy.bit2me.com/en/que-es-un-hackeo-finney-ataque-finney/