Unchecked Low Level Calls

// VULNERABLE: Return value ignored
function withdraw(uint256 amount) public {
    balances[msg.sender] -= amount;
    payable(msg.sender).call{value: amount}("");  // Silent failure!
}

// VULNERABLE: Delegatecall without check
function upgrade(address impl) public onlyOwner {
    impl.delegatecall(abi.encodeWithSignature("initialize()"));
}

// VULNERABLE: ERC20 transfer without check
function distribute(address token, address[] memory recipients) public {
    for (uint i = 0; i < recipients.length; i++) {
        IERC20(token).transfer(recipients[i], 100);  // May fail silently
    }
}

// SAFE: Explicit check
function withdraw(uint256 amount) public {
    balances[msg.sender] -= amount;
    (bool success, ) = payable(msg.sender).call{value: amount}("");
    require(success, "Transfer failed");
}

// SAFE: SafeERC20 library
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

using SafeERC20 for IERC20;

function distribute(address token, address[] memory recipients) public {
    for (uint i = 0; i < recipients.length; i++) {
        IERC20(token).safeTransfer(recipients[i], 100);  // Reverts on failure
    }
}

// SAFE: transfer() reverts on failure (but limited gas)
function withdraw(uint256 amount) public {
    balances[msg.sender] -= amount;
    payable(msg.sender).transfer(amount);  // Reverts if fails
}

{
  "issues": [
    {
      "tag": "unchecked_call",
      "severity": "high",
      "description": "Low-level call without return value check",
      "location": "withdraw(uint256):L45"
    }
  ]
}