ERC777 tokens allow arbitrary callbacks via hooks that are called during token transfers. Malicious contract addresses may cause reentrancy on such callbacks if reentrancy guards are not used.

• DAO Hack
  • Token: ETH
  • Trigger: Using the fallback function
    • this is executed when receiving ETH by contract
• ERC777 Hack
  • Token: ERC777 Token
  • Trigger: tokensReceived()
    • this is executed when receiving the ERC777 token by contract

address token;
mapping(address => uint) canBorrowAmount;
/* 🙅‍♂️ */
function badBorrow(uint amounts) public {
		require(amounts <= canBorrowAmount[msg.sender]); // 1.𝗖𝗛𝗘𝗖𝗞
    IERC777(token).transfer(msg.sender, amounts) // 3.𝗜𝗡𝗧𝗘𝗥𝗔𝗖𝗧𝗜𝗢𝗡
    canBorrowAmount[msg.sender] - amounts; // 2.𝗘𝗙𝗙𝗘𝗖𝗧𝗦
  }

/* 🙆‍♂️ */
function goodBorrow(uint amounts) public {
		require(amounts <= canBorrowAmount[msg.sender]); // 1.𝗖𝗛𝗘𝗖𝗞
    canBorrowAmount[msg.sender] - amounts; // 2.𝗘𝗙𝗙𝗘𝗖𝗧𝗦
    IERC777(token).transfer(msg.sender, amounts) // 3.𝗜𝗡𝗧𝗘𝗥𝗔𝗖𝗧𝗜𝗢𝗡
  }

https://eips.ethereum.org/EIPS/eip-777

https://twitter.com/dmihal/status/1251505373992845317

https://medium.com/amber-group/preventing-re-entrancy-attacks-lessons-from-history-c2d96480fac3