Comment 5 for bug 1272438

I've seen the lockup from kwin using 100% cpu on two different Kubuntu 14.04 machines, without needing to switch users. These machines were ungrades from previous versions, 12.04 and 13.10.

I think its fixed by resetting the kwin configure file by:

run "System Settings"
select "Configure desktop effects"
then clicking "Defaults" (at the bottom) and then "Apply" (at the bottom right).

If you can get to term with Alt-Ctrl-F1, you can use "DISPLAY=:0 kwin --replace" to fix a broken kwin using 100% cpu. This also works if you can "ssh" in.