Notaries can be bribed, sure, but smart contracts can and will have bugs.
This is one of the fundamental problems with blockchains: Local problems often lead to global consequences, requiring forks of the blockchain and/or high level intervention to maintain consistency or to roll back exploits. Here's the classic example:
https://www.coindesk.com/learn/2016/06/25/understanding-the-...
'Don't write bugs' is not a viable strategy for software development.