New Case with AllowMultipleCaseClients enabled throw form rule error (please select a valid contact) OR notice (undefined constant)

There are missing single quotes in variable in CRM/Case/Form/Activity/OpenCase.php. In CiviCRM 4 / Drupal 7, the stricter error checking either throws an warning or triggers the formRule incorrectly. Fix is to add missing single quotes on line 228:

replace this:
if ( isset( $fields[contact][1] ) && !$fields[contact][1] ) {
with this:
if ( isset( $fields['contact'][1] ) && !$fields['contact'][1] ) {

http://forum.civicrm.org/index.php/topic,20175.0/topicseen.html