To test the problem described in the title of this task.
Open any document in Edit mode in the Page namespace of Wikisource, and click on "Preview". Each "Preview" action appends and empty row. Since a page sometimes needs to be previewed 5-6 times, this adds 5-6 empty rows at the end of the text.
As far as I could ascertain, this doesn't seem to affect the appearance of text transcluded to the main namespace, nevertheless, this confuses editors when a page needs to be re-edited later.