Current group don't saves in $SESSION (value destroys after page will be reloaded)
Current group may be changed to other group or destroyed by "false" value if user not in any groups (like teacher)

Index: group/lib/legacylib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/group/lib/legacylib.php,v
retrieving revision 1.6.2.3
diff -u -r1.6.2.3 legacylib.php
— group/lib/legacylib.php 6 Mar 2007 04:28:45 -0000 1.6.2.3
+++ group/lib/legacylib.php 14 May 2007 18:31:19 -0000
@@ -210,11 +210,12 @@
$currentgroup = $SESSION->currentgroup[$courseid];
} else

• if ($currentgroup) { $SESSION->currentgroup[$courseid] = $mygroupid; }

  +
  +// if ($currentgroup)

  { // we should not change or break $currentgroup if it was set (dlnsk) +// $SESSION->currentgroup[$courseid] = $mygroupid; +// }

if ($full) {
return groups_groupid_to_group($currentgroup);