During an upgrade from 1.6.1+ to 1.6.2 I had a critical bug in hotpot module saying something about MetaColumns() as a non-object member.
Here is the patch:

Index: mod/hotpot/db/postgres7.php
===================================================================
RCS file: /cvsroot/moodle/moodle/mod/hotpot/db/postgres7.php,v
retrieving revision 1.16.2.3
diff -c -r1.16.2.3 postgres7.php

• • • mod/hotpot/db/postgres7.php 13 Sep 2006 08:26:16 -0000 1.16.2.3
    • mod/hotpot/db/postgres7.php 13 Sep 2006 11:56:13 -0000
      ***************
    • 1,6 ****
      <?PHP
      function hotpot_upgrade($oldversion) {
      ! global $CFG;
      $ok = true;

// if the version number indicates this could be an early HotPot v2.1 (Moodle 1.6),
— 1,6 ----
<?PHP
function hotpot_upgrade($oldversion) {
! global $db, $CFG;
$ok = true;

// if the version number indicates this could be an early HotPot v2.1 (Moodle 1.6),