Ask HN: How to fix an error on a blockchain?

We are notoriously bad at writing bug-free, secure code and yet blockchain advocates suggest that we can deploy immutable smart contracts and let this code (written by imperfect developers) be the source of truth. The most notorious example of this the "The DAO hack".

Bad coding practices is not the only problem. People can accidentally or intentionally enter incorrect data, discover new edge cases after an immutable code was deployed, decide to change a logic in smart contracts, etc.

What is suppose to happen when errors in smart contract/blockchain applications/data on a blockchain occur?