It would be helpful for the junior folks among us (and those still learning) if you explained a bit about why that code is so terrible.
Well first off.
IsNegative and Balance are idependantly mutable.
I could do this:
var account = new Account();
account.Balance = 500;
account.IsNegative = true;
var account = new Account();
account.Balance = 500;
account.Credit(-600);
Or vice versa for both.
I would expect anyone applying for a job that is asking for 5+ years of experience to pick up on something as basic as this.
OK... so you're complaining that the coder could use the api to violate an invariant. I've worked with millions of lines of code and this is certainly not a criterion I'd use to evaluate a serious programmer. Many codes I work with permit this sort of behavior (I guarantee you can mess up using the C library this way).