when you try change the owner group of a file with thunar (root>your_user) the list of groups clear automatic :-/
Fixed with revision 21590. 2006-05-08 Benedikt Meurer <benny@xfce.org> * thunar-vfs/thunar-vfs-user.c(thunar_vfs_user_manager_get_all_groups): Properly rewind/close the groups database before iterating over the groups. Bug #1772.