AutoCAD 2010 :: Hatch Creating Error In DWG File - Lines Disappearing

We are using a file created by another company and making additions to it for Interiors. While working in this file we have added some new layers and have begun to hatch new walls. While hatching some of these wall my boss received a message saying that the file had errors. He saved and closed the file at that point. When he re-opened the file a number of lines were missing from the file.

They appear to be completely random - i.e. parts of chairs are missing but not the whole chair, parts of walls but not whole walls. The things missing were not all on the same layer and were not necessarily items that had been hatched.

Note - we are running 2012 and the files were created in 2007