In my volunteer mode, I was removing content that was copied from an external site. I didn't add any content at all, but simply deleted text (and a template) and merged the resultant material into a single paragraph.
VisualEditor flagged this as my adding wikimarkup and gave me the nowiki warning. :)
See http://en.wikipedia.org/wiki/File:Wikimarkup_warning_without_case.png which is confusingly titled, because I missed the "u" in "cause."