Cardano: "Slow and Steady wins the... Annnnnd it's gone." Plutus auditor publicly announces that most Cardano smart contracts have an exploit.

https://www.canonicalllc.com/post/psa-do-this-and-prevent-your-dapp-from-getting-hacked

If you do not understand the nature of the attack, there is a high likelihood you have it in your code. Additionally, mitigating the attack has ramifications around the design and efficiency of smart contracts, which can lead to considerable amounts of redesign if you are not aware of the design constraints early on.

Since Mid-October, every Cardano dApp with publicly accessible smart contract code, has had a similar exploit in their initial smart contract.

Privately, Canonical has reached out to SundaeSwap, MLabs, Well-Typed and IOHK to raise awareness of the issue, and to attempt to prevent new dApps from including it.

A full in-depth description is coming. Additionally, Canonical has been building consensus privately among the Plutus developer community on how to prevent this exploit, initially through best practices, and ultimately via design changes to the Plutus smart contract system.

Edit: bold emphasis added to the words the Cardano cult is most likely to ignore.