In windchill 10.2 M010, Create an Enditem and attach a document and cad document in the related tab of the end item.
Go to the structure tab of an enditem --> Click on Show/hide option --> select document/cad document, so that both will be visible in the structure.
Click on the document and check out.Again do undo checkout of same document. Here the check in happens successfully but gives an error of " The object with the following key was deleted or inaccessible: "wt.part.WTPartDescribeLink:xxxxxxxx" .
Since this functionality is useful to checkout and edit /Undo checkout the document in the structure tab itself it may be considered as an important option.
If the show/hide functionality is only for listing the related objects then the checkout functionality should be disabled when clicked on the documents.