All simple, project-based compound clips created on the timeline. For example, I create a title, add a transition at the head and then create a compound clip. Later, when I need to adjust the title, I can’t break it apart. There doesn’t seem to be a rhyme or reason to when this happens. Some of the quirks I’ve encountered with FCPX will disappear with a reboot – not this one, though.
This has been a typical part of my workflow since I’ve been using FCPX. I don’t think I’m doing anything differently than I was prior to 10.0.3.