Compound Clip - strange behaviour

I have several "replace screen" clips where I'm, tracking a hand held device that's really not moving that much using keyframes on the transform section. It's working well.

Having got al this working I decided to create compound clips so the original and replacement screens are one. The original purpose for doing this was because I wanted to do some retiming of these particular clips.

Two of the three times I created a compound clip everything went perfectly. I'm selecting both top and bottom clip within the project timeline, hitting Alt-G, naming it and voila... a compound clip.

On the third one however the compound clip that's created has only one clip in it, and it's not even one of the two selected, it's a completely different clip.

Has anyone come across this before? Have I miss understood how to create compound clips for the last couple of years and I'm doing it all wrong, or is it that FCPX is throwing a wobbly and maybe something is corrupt?

Wew that's weird. Can you post a screenshot of the two clips on the timeline? Are they different in any way from the clips you have used to create the first two CCs?

- Ronny