"Choose when to turn off the display" Bug

I have my monitor set to turn off after a period of inactive time. However sometimes this is what happens

http://img26.imageshack.us/img26/7302/img18391h.jpg

Black screen with mouse pointer.

Now this only seems to happen if I have an internet page that has some kind of Adobe Flash going on. Does anyone know a fix othe than simply exiting Firefox before leaving my PC?