GNU Screen is great. So great that I find myself always using it. (Pressing the Window key and T launches an XTerm with a new Screen ready on my system, while I have to add Shift if I don’t want the screen.) This means that when I login into a some other machine through SSH—an occasion for which Screen is particularly useful—I will often end up with nested screens. So which Screen will receive my Ctrl+a presses?
Not that I don’t still find controlling nested screen confusing, but now at least I don’t feel helpless and stuck whenever it happens. 😉