Just never sign a transaction unless its locktimed for at least 5 blocks in the future. TADA "undo". Enjoy.
I like it! OP could even patch one of the open source wallets to do this for him. I won't use it though: it'll just be annoyingly slow. Waiting for 1 confirmation already feels slow when I order something.more forgiving for human errors
Scammers will love this feature!While bitcoin is the gold standard of cryptocoins, it is indeed slow, the idea is that other faster blockchains/dagchains will overtake/also incorporate this feature so that faster blockchains can also support it, but maybe with more "grace" blocks, otherwise the undo time would be quite limited.
Concerning the scammers
It's good practice for people to wait a certain number of transactions for confirmations =D This could also be build into software to make sure transactions weren't undone.