It is possible to drag an applet below the taskbar and hide it completely.

STEPS TO REPRODUCE
1) Open an applet into any browser (plugin2 supported browser)
2) Drag the applet to detach it from browser (to drag use ALT+left click)
3) Keept the mouse cursor at the top most position of the applet and drag it below the taskbar as much below you can
 

ACTUAL BEHAVIOUR
The applet completely goes behind the taskbar and gets hidden

EXPECTED BEHAIOUR
The movement of the applet should be restricted to some point from where it is possible to drag it back.