There are two errors that might occur. In both cases an error message should be shown.
- Extends points to a reference, that cannot be found (e.g. does not exist, not yet defined).
- Extending more than one level.
The wish to improve the referencing of multiple parts of the same source was part of the German-speaking community's technical wishlist in 2013, 2015 and of the international technical wishlist in 2015. For more info, see T100645.