What happens if I leave a creditor out of my Bankruptcy?

It depends on what time of bankruptcy was filed and if you have assets.  In a chapter 13 bankruptcy and in a chapter 7 bankruptcy where there are assets, the debt will not be discharged.  Which means you will still be on the hook to pay the bankruptcy,

In a no-asset chapter 7 bankruptcy, the debt will be discharged as long as it is one of the normal types of debt forgiven in bankruptcy like medical debt, credit cards, auto loans for reposed cars and the like.