For example this URL causes an error:

http://localhost/~sasha/zabbix-svn/trunk/frontends/php/hostgroups.php?sort=flags

Sorting by field "flags" not allowed. [hostgroups.php:333 ? CFrontendApiWrapper->get() ? CApiWrapper->__call() ? CFrontendApiWrapper->callMethod() ? CApiWrapper->callMethod() ? CFrontendApiWrapper->callClientMethod() ? CLocalApiClient->callMethod() ? call_user_func_array() ? CHostGroup->get() ? CApiService->applyQuerySortOptions() in api/classes/CHostGroup.php:344]
array_slice() expects parameter 1 to be array, boolean given [hostgroups.php:335 ? getPagingLine() ? array_slice() in include/func.inc.php:1574]