Create a page that contains refs with mis-matched content:

<ref name="foo">Hello</ref>
<ref name="foo">World</ref>

Parsoid will render this fine (show preview in NWE). However, when you save the page, you get a red error message that says Cite error: Invalid <ref> tag; name "foo" defined multiple times with different content

Reported by User:Miangara at https://www.mediawiki.org/wiki/Topic:Thqinkkzksn0gcue about the errors corrected in these edits: https://es.wikipedia.org/w/index.php?title=Tour_de_Francia_2009&diff=96007668&oldid=93654807