Right, so what happens is you saved your session in karmic and that saved "compiz.real" as a program to run in your session. In lucid compiz.real doesn't exist so gnome-session tries to start it and fails. Ideally gnome-session would then just start the default WM ("compiz") but it does not. That's why a workaround is to either remove your saved session file or get compiz running then save your session again.
Right, so what happens is you saved your session in karmic and that saved "compiz.real" as a program to run in your session. In lucid compiz.real doesn't exist so gnome-session tries to start it and fails. Ideally gnome-session would then just start the default WM ("compiz") but it does not. That's why a workaround is to either remove your saved session file or get compiz running then save your session again.