Quick catch, Nibbler. I cannot figure out what width="100%%" means (and obviously didn't know at the time I made the change). With a single percent, the line never appears in the output, and you have the reported bug (which I apologize for not seeing). With two percent signs, the line is output and it only has one percent sign. It is single-quoted in the array, so it's not variable interpolation. And I cannot think of any other reason it's necessary. And I cannot figure out how to search for it on google or php.net.
Regardless, it works, so here's the fix. Look for this variable in include/themes.inc.php and put in two percent signs as shown:
// Template used for tabbed display
$template_tab_display = array('left_text' => '<td width="100%%" align="left" valign="middle" class="tableh1_compact" style="white-space: nowrap"><b>{LEFT_TEXT}</b></td>' . "\n",
'tab_header' => '',
'tab_trailer' => '',
'active_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="tableb_compact"><b>%d</b></td>',
'inactive_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>%d</b></a></td>' . "\n",
'inactive_prev_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>{PREV}</b></a></td>' . "\n",
'inactive_next_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>{NEXT}</b></a></td>' . "\n",
);
You can alternatively add this variable into themes/yourtheme/theme.php and not touch the core script.