What does this contract do? Can you spot a bug?

Puzzle.sol

pragma solidity ^0.4.0;

contract Puzzle {
    address _a;
    address _b;

    constructor(address a, address b) public {
        _a = a;
        _b = b;
    }
    
    function () public payable {
        _a.transfer(msg.value / 2);
        _b.transfer(msg.value / 2);
    }
}