I was playing with how to enter unicode on plasma. I went into the Input Devices KCM->Advanced, and by default "Configure keyboard options" is unchecked. I selected that, then selected Position of Compose Key->Left Win (that is the only thing selected). After I was done playing, I decided I no longer wanted that enabled, so I went back to KCM and de-selected the Compose Key, and for good measure I also de-selected the "Configure keyboard options" and clicked apply. After this, I could still use the compose key. The only way I found to make it stop, was to log out/log in. Reproducible: Always
This bug is still reproducible on current master. Weirdly though, Meta (Win key) continues to work to open the menu in addition to functioning as Compose. After logout and relogin, it does not work for opening the menu, but it does work as Compose.
*** This bug has been marked as a duplicate of bug 380887 ***