Could that be done? With all the cheque scams, I'm guessing yes: someone cashes a bad cheque into your account, suddenly it says you have, say, 10K more money, and then you get a phone call from a "lawyer" claiming please return the money to some other account. You do so, and then the bank tells you the cheque didn't clear, they're taking 10K away from your account...

If a crypto (or other digital) currency has reversible transactions ("Undo this transaction and all the after effects"), that would be cool. So if I spent that 10K on a new Mac, after the Undo I'd have the Mac and a huge unpaid bill from the Apple Store...