Example.sol

pragma solidity ^0.8.0;

contract Example {
	uint256 public totalDeposits;
	mapping(address => uint256) public deposits;
	
	function deposit() external payable {
		deposits[msg.sender] += msg.value;
		totalDeposits += msg.value;
	}
	
	function withdraw() external {
		/*
		Apparently the only way to deposit ETH in the contract is via the `deposit` function.
		If that were the case, this strict equality would always hold.
		But anyone can deposit ETH via selfdestruct, or by setting this contract as the target
		of a beacon chain withdrawal.
		(see last paragraph of this section
		https://eth2book.info/capella/part2/deposits-withdrawals/withdrawal-processing/#performing-withdrawals),
		regardless of the contract not having a `receive` function.
		
		If anybody deposits ETH that way, then the equality breaks and the contract is DoS'd.
		To fix it, the code could be changed to >= instead of ==. Which means that the available
		ETH balance should be _at least_ `totalDeposits`, which makes more sense.
		*/
	  	assert(address(this.balance) == totalDeposits); // bad
		
		uint256 amount = deposits[msg.sender];
		totalDeposits -= amount;
		deposits[msg.sender] = 0;
		
		payable(msg.sender).transfer(amount);
	}
}