To whom it may concern.
It looks like https://bugs.launchpad.net/ubuntu/+source/gnome-panel/+bug/44082 has been resurrected itself.
I have noticed it in the upper panel when using traditional panel layout. As soon as 'expand' flag in the panel properties is cleared and gui session is rebooted applets change their placing in the panel. Restoration of applets' order is almost impossible until a sort of 'reverse rule' is noticed. Namely: an applet which is moved last is displayed first. Nevertheless, applets sometimes jump along the panel randomly at their own will.