Links that work are forward links like this one in file xlec5.html:
<h4><a id="lnk0" class="butn" href="zguide5.html#lnk0">+</a></h4>
but those that trigger footnotes are the return links which look like this one in file zguide5.html:
<p><a id="lnk0" class="buto" href="xlec5.html">+</a><b>Fifth Lecture</b></p>
This causes file xlec5.html to display in the footnote window instead of jumping back to the top of chapter 5 in the main display.