Only debts that are listed on your petition will be discharged in bankruptcy. Additionally, when you file a petition for bankruptcy, you are swearing under oath that you have included all of your debts and assets. Intentionally omitting a creditor is actually perjury.
Of course, you should exercise extreme care to ensure that you list all of your debts on your bankruptcy petition. However, there can be circumstances where an individual does not know about a creditor at the time of filing.
If you accidentally leave a debt off of your bankruptcy petition, the bankruptcy court will allow you to file an amendment to your schedules. As long as the amendment is filed in a timely manner, then the creditor will be added to your bankruptcy.
Note, though, that there are time limits on how long you have to file amendments. These time limits are strict and specific. That is why you should immediately seek the advice of a qualified bankruptcy attorney to ensure that you do not lose out on any discharges to which you would otherwise be entitled.