I have observed this behaviour when using Eclipse Mylyn.

(1) Install eclipse and eclipse-mylyn-*
(2) Open any bug that is on QA state.
(3) Try to move it to 'CLOSED'
(4) Error appears that indicates bugzilla error:

(Eclipse says) Error may result when QAContact field not enabled.
((Embedded browser shows)
 Undef to trick_taint at Bugzilla/Util.pm line 67
       Bugzilla::Util::trick_taint('undef') called at Bugzilla/Bug.pm line
 4525
       Bugzilla::Bug::LogActivityEntry(766608, 'cf_last_closed', 'undef',
 '2012-04-17 07:22:48', 324097, '2012-04-17 07:22:48') called
 at ./extensions/RedHat/Extension.pm line 992
 
 ugzilla::Extension::RedHat::bug_end_of_update('Bugzilla::Extension::RedHat=HASH(0xa930c0)', 'HASH(0xe243760)') called at Bugzilla/Hook.pm line 33
       Bugzilla::Hook::process('bug_end_of_update', 'HASH(0xe243760)') called
 at Bugzilla/Bug.pm line 1157
       Bugzilla::Bug::update('Bugzilla::Bug=HASH(0xddf60d0)') called
 at /var/www/html/bugzilla/process_bug.cgi line 498