The Security of Smart Contracts

The security of smart contracts, or how can smart contracts actually fail? What's the problem with them? So far we've just spoken about how neat they are and how many croel things they can do. But i think that goes back to the different use cases that people imagine. And i mean, i've heard arguments that you should not touch smart contracts unless you're willing to like do, like, write your entire smart contract and by code and formally prove that it's correct to your intention.