When the caption for a file in a <gallery> contains a <ref> with a group, the resulting page contains an error message produced by Cite.php. This can be seen in http://parsoid-lb.eqiad.wikimedia.org/enwiki/Klondike_Gold_Rush?oldid=651957368 at the end of the "Beginning of the stampede" section.
It can also be reproduced on the command line:
$ echo -e "<gallery>\nFile:foo.jpg|A <ref group="n">ref one</ref>\n</gallery>" | node parse … <ul class="gallery mw-gallery-traditional" typeof="mw:Extension/gallery" data-mw='{"name":"gallery","attrs":{},"body":{"extsrc":"\nFile:foo.jpg|A <ref group=n>ref one</ref>\n"}}' data-parsoid='{"dsr":[0,62,2,2]}' about="#mwt3"> <li class="gallerybox" style="width: 155px"> <div style="width: 155px"> <div class="thumb" style="width: 150px;"> <div style="margin:35.5px auto;"><a href="/wiki/File:Foo.jpg" class="image"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/0/06/Foo.jpg/120px-Foo.jpg" width="120" height="79" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/06/Foo.jpg/180px-Foo.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/06/Foo.jpg/240px-Foo.jpg 2x" data-file-width="300" data-file-height="197"/></a></div> </div> <div class="gallerytext"> <p>A <sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span>[</span>n 1<span>]</span></a></sup></p> </div> </div> </li> </ul><span about="#mwt3"> </span><p about="#mwt3"><br/> <strong class="error mw-ext-cite-error">Cite error: There are <code><ref group=n></code> tags on this page, but the references will not show without a <code>{{reflist|group=n}}</code> template (see the <a href="/wiki/Help:Cite_errors/Cite_error_group_refs_without_references" title="Help:Cite errors/Cite error group refs without references">help page</a>).</strong> <link rel="mw:PageProp/Category" href="./Category:Pages_with_missing_references_list#Main%20Page"/></p>
Could this be caused by using the <gallery>'s contents as a whole page, thus confusing Cite.php?