This is valid code in 0.4.21:

contract ClientReceipt {
    event Deposit();
    function deposit() public payable {
        uint256 emit;
        emit Deposit();
    }
}

The key thing is that the emit statement is enabled from 0.4.21, but emit is still valid as an identifier. We currently cannot handle this, and it makes it impossible to compute over the versioning of tokens vs. identifiers.