List of steps to reproduce (step by step, including full links if applicable):

• edit a source text page
• switch to "Advanced" in wikitext editor
• use find-replace tool (icon at top right, with java popup window

(see https://en.wikisource.org/w/index.php?title=Page:An_Essay_on_the_Opera%27s_After_the_Italian_Manner.pdf/11&diff=prev&oldid=11608000&diffmode=source for example)

What happens?:
Wikitext that belongs the in the page header and footer is duplicated and appended to the text in the page body.... find-replace doesn't understand Page namespace, apparently. The intended replacement IS made correctly, including in the appended header and footer (though not in the 'actual' ones)

What should have happened instead?:
Umm, obvious much? :)