Error Message, Part or Assembly will not Save, or other issues.
SolutionWithin a single part or in the top most level assembly file, select the following:
File (tab) -> Utilities -> Part Cleanup -> select work part & components
Select all check boxes with the exception of Not selecting those in the Serious Delete Actions section. (see image below)
Also set Empty Groups option to Delete All. (see image below)
This is NX's repair tool and it will check and correct problems within a single file or within all files and even inter-part relationships within an assembly of part files.