Typically, in a Chapter 11 case, if a creditor does not file a proof of claim prior to the claims bar date, then they can lose their right to any pre-petition debt they might be owed, particularly if the debtor had no knowledge of the obligation, or potential obligation and thus did not include the creditor in the filing.  Post-petition obligations would be another matter, and of course there are a number of exceptions related to various notification issues.  In the grand scheme of things this is a very minor matter; sometimes a fish shakes the hook.