I've had a friend confirm a similar issue (he removed one user from the admin group, and other users were removed as well), but I never ended up having a look at his system to see what went wrong.
** Changed in: Ubuntu Sourcepackagename: None => gnome-system-tools -- Removing one sudo user removes the sudo privilege of another sudo user https://launchpad.net/bugs/51201 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
