Mysterious insertion of paragraph tag (in DOM only, not in code) inside span

.section-aboutus .date-display-single {
  color:#2d95c1;
}

.section-aboutus .date-display-single p { margin:0px; padding:0px; }

Can that second CSS rule call the p tag into being?

Definitely one of the freakiest things I've seen. The source code contained no p tag within the span. The code shown by Firebug and Safari's inspector both had the the paragraph tag.

Actual source code:

<p>Tri Star Pictures <span class="date-display-single">2009</span></p>

Browser's interpretation as revealed by Firebug and Inspector:

<p>
  "Tri Star Pictures "
  <span class="date-display-single">
    <p>2009</p>
  </span>
</p>

(inline fields don't go through the field template, so not quite sure where to theme them)
http://www.advantagelabs.com/drupal-colonoscopy-or-how-theme-cck-field-drupal-6

Ended up stripping span tags from around year output (strip_tags right in the print statement) to get around the weirdest HTML/CSS bug I've ever seen where paragraph tags are actually being inserted, unasked, into the DOM.