Kill switches
When writing contracts, be especially careful to write secure code and include a kill switch to ensure that if any bugs do reside in the code, they can be squashed. If you do not include a kill switch and there are vulnerabilities in the smart contract that can be exploited, this can lead to the theft of resources from the smart contract or from other individuals.
This was brought into sharp focus during the infamous DAO incident. The DAO smart contract acted as a complex, decentralized venture capital fund and held Ether worth $250 million at its peak collected from a group of investors. Hackers exploited vulnerabilities in the smart contract, stealing $50 million worth of Ether.
Because there is no way to undo transactions in Ether, there was a highly controversial “hard fork," where the majority of the community agreed to accept a block that contained an irregular state change that essentially drained all DAO funds into a special “WithdrawDAO” recovery contract. By convincing enough miners to accept this irregular block as valid, the DAO was able to return investors funds. However, not everyone agreed with the chain, with those who disagreed rejecting the irregular block and forming the Ethereum Classic network, each blockchain grew independently.
Limitations
Kill switches can cause their own problems. Like if a contract that is a library has its kill switch flipped. All contracts relying on this contract can’t operate as intended even though the underlying library code is immutable. Recently, an attacker triggered a kill switch in an underlying library function that caused over 500,000 Ether to get stuck in multi-signature wallets. Users of the multi-signature library assumed the immutability of the code meant that the library would always operate as anticipated, and accepted the block. In the wake of this, there are many tools that check smart contracts for bugs or enable bug bounties.
Smart contracts interacting with the blockchain are only deterministic when accounting for the state of the blockchain.
How is this different?
This is a radically different approach for providing transparency and accountability. Because all contracts and transactions are public and verified by consensus, trust is distributed amongst the people, rather than centralized in a few big institutions.
The trust given to institutions is historic. This history builds trustworthiness.
The trust placed in consensus-based algorithms is based on the assumption that most people are honest, or more accurately that no sufficiently large subset of people can be made to collude to produce a malicious outcome. This is the democratisation of trust. In the DAO attack, a majority of nodes agreed to accept an irregular state transition. This effectively undid the damage of the attack, and shows how, at least in the world of blockchain, perception is reality. Because most people “believed" – accepted this irregular block, it became a “real” – valid block.