Re: disallowing claims
BK doesn't deal with debt but rather claims. A claim is just that, a claim. The claim must be valid and have lien rights against the debtor's property (even if unsecured). If the claim is not a valid lien right for debtor property, then it might not exist. Even if a claim meets all the above criteria, it might be disallowed as a preference payment, a fraudulent transfer, etc.
If i recall correctly, during the Iridium filing the secured lenders did not properly perfect their liens and became a large, unsecured lender. In another case, i saw a lender forget to perfect liens on real property (90% of assets in case) and therefore suffered reduced recovery.