I saw a chain, I think it was swingby that implemented a transaction undo.
Use case 1:
Sally wants to send betty 1btc to help fund with a potentially terminal illness.
Sally sends it, but Betty dies before the block is done validating.
Sally is sad, but she can still retrieve her funds.
Use case 2:
Bills computer has a virus that sometimes copy pastes extra characters, or his screen is broke and randomly glitches and hits send.
Bill wants to transfer funds between his two wallets. from wallet a to wallet b.
Bill sends his funds from a.
In his other wallet b, if he doesn’t see the funds he can go back to wallet a and hit undo. But if he does see his funds in wallet b with a button “secure received funds” he hits it and now it is in wallet b.
Basically a smart contract escrow that either side can affect the money.