Transparent windows: how to get rid of it?

[Freddy Van Ingelgom]

I use Ubuntu 11.10 with Unity as interface. Ican not say that I was always
happy with this Unity thing, but I like it now. There is one thing I hate:
from time to time there is a sort of transparent window appearing on the
screen and I can not find a way to get rid of it! I would like to know what
the purpose of this "window" is, what triggers the appearance of this
window, and most of all: is there a way to switch this "feature" off?
Any help is most appreciated!

Freddy
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