TImeline clip with a shadow

Once in a while I notice that in my timeline one of the clips, for some reasons I don't understand, has become kind of selected. It is almost the same as others but has a shadow. That shadow doesn't go away when the timeline is played or edited. I wonder what could that be? Here I also attach a screen shot of it, the clip with the shadow is in the middle.

At this point you can call it a "bug". Normally it goes away when you quit FCPX and launch it again. There have been some speculations that this bug may turn out to be a feature in a future version, but I'm not a speculations guy

- Ronny