There have been many incidents in DeFi where protocols lost millions of dollars accidentally.
In Parity Hack, Unlike most cryptocurrency hacks, however, the money wasn’t deliberately taken: it was effectively destroyed by accident. The lost money was in the form of Ether, the tradable currency that fuels the Ethereum distributed app platform, and was kept in digital multi-signature wallets built by a developer called Parity. These wallets require more than one user to enter their key before funds can be transferred.
Another case was with OptiFi, where they accidentally closed the OptiFi mainnet program and it was not recoverable. The total value locked on the platform, amounting to $661,000 was now locked.
https://twitter.com/OptifiLabs/status/1564367455220219904?t=TmZkGEEzsWunCGv0-KX1hg