Some tables in Moodle put the table heading inside the tbody section rather than the thead. As well as being incorrect, this has at least two pragmatic effects. 1) It makes Moodle harder to theme, 2) it affects accessability.

Some examples are:

table#modules /admin/modules.php
table#partipants /user/index.php?id=1
table.compatibleblockstable /admin/blocks.php
table.minicalendar /calendar/view.php?view=month&cal_d=1&cal_m=7&cal_y=2012&course=1

There might be more, adding the following lines to a theme should highlight the issue as you browse the site, the second line hides a common false positive for vertical tables with headers as the left column:

table tbody tr th

table tbody tr th:first-child