I think I've had this bug - although I'm not sure what Compiz is. For me, the whole unity interface hangs and disappears, leaving the current window(s) open. Sometime alt_tab will trigger this, sometime not. I also noticed it would trigger on logging out while any windows were still open. On login, windows are left open but Unity (launcher, top menu bar) are gone.
I think I've had this bug - although I'm not sure what Compiz is. For me, the whole unity interface hangs and disappears, leaving the current window(s) open. Sometime alt_tab will trigger this, sometime not. I also noticed it would trigger on logging out while any windows were still open. On login, windows are left open but Unity (launcher, top menu bar) are gone.