UnexpectedEther.sol

/* Bad */
uint256 constant private targetEther = 1000 ether;
function join() public payable {    
    require(msg.value == 5 ether); // each play is 5 ether
    ...doSomething;
}

function claimReward(address _to) public {
	require(this.balance == targetEther);
	_to.transfer(targetEther);
}

/* Vulnerable
1. Send ether by selfdestruct
2. the value of this.balance cwill never be a multiples of 5 forever...
*/

/* Better */
uint256 constant private targetEther = 1000 ether;
uint256 private treasury;
function join() public payable {    
    require(msg.value == 5 ether); // each play is 5 ether
    treasury = treasury + 5 ether;
    ...doSomething;
}
function claimReward(address _to) public {
	require(treasury == targetEther); // don't use this.balance
	_to.transfer(targetEther);
}