Visual Editor moves ProofreadPage haeader / footer into page text field, duplicating them.
Recent example of such edit:
https://pl.wikisource.org/w/index.php?title=Strona%3AM._Arcta_S%C5%82ownik_Staropolski.djvu%2F0354&type=revision&diff=2440000&oldid=1973406
Note: this should no longer be a problem for plwikisource, as similar edits are now blocked by Abusefillter. But this may be problem for VE users who may be not able to edit until this is fixed.