bad credit history

I don't think they're allowed to discriminate based on your credit history or financial status. At least in the United States.

Some jobs like Department of Defense (or Federal/State jobs) that require a security clearance may require you to keep your Debt-to-Income ratio low (I think its below 18%?) or they can have your access suspended and investigate your situation. They don't want financially insecure persons holding a security clearance. They fear it could make you more likely to sell secrets. And they are perfectly justified to fire you for it. However, if they're smart they'll do a full investigation because if you file a complaint or sue they'll find themselves in a discrimination case if they can't support their claim.

Other than that, I can't think of anything.