We routinely hear stories about a "fat fingered" trader. While most of the trades remain irreversible, sometimes they can be rolled back too. What happens if there is a massive error (or even a hack) and the trades of say 1billion cannot be rolled back?

The problem of immutability works both ways. Surely, DTCC could build a better solution, provide say better APIs (hmmm, startup idea if someone wants to take it up) to brokers so that the information is always up to date instead of relying on blockchain.