Today, while working on a multiclip started by another editor one of the clips in a multiclip segment was visually blank. The audio was there, and it played fine in QT Player but just before the In point (in the Viewer) which was set it would go blank.
An odd behavior was that dragging this angle to the timeline only showed about 1min's worth of footage rather than the full 4+ minutes of the clip. While I'm still not sure what is going wrong -it must have been visible when the original editor was working with it since it's synced up as a multiclip and cut into the Timeline- I solved it by doing basically two things:
01) I simply duplicated the clip in the Finder by Option + Dragging it (old habit, Command + D would have also worked).
02) After appending the name of that clip with "_COPY_", I brought it into FCP's bin, set an identical In point as the original bad clip, then dragged it to the Multiclip open in the Viewer atop the bad angle and waited for moment for a contextual menu to pop up and selected Overwrite Angle.
If you don't see the Overwrite Angle option, check to make sure that the angle you're trying to overwrite isn't currently used (displayed) in the Canvas via the Timeline. You'll have to switch to another angle, Overwrite Angle, then switch back.
Keep an eye on your audio track sources if you encounter this problem and use this solution. They may still come from the "bad" angle clip.
Then I saved the project and everything seemed fine. I'll update this if I find out exactly why FCP suddenly doesn't like that one multiclip angle.
Another potential solution, which didn't work for me, was deleting the clip from the bin and re-dragging it back in. One last thing you can try is to simply re-import the clip which I unfortunately don't have the option of trying with this project.
Update: Another thing to try, and this just worked for me on a project, was to close the project, reopen it, and then Re-Connect all of the media. Maybe just reconnecting the errant media in the timeline would also work. But, it's another thing to try.
The important thing to remember is that you shouldn't get any mis-match errors about "media start and end" when you try and reconnect your footage. If you see that message, hit Try Again and...try it again.
In the reconnect window "Amount with conflicts" should read 0 (zero) if you're trying this fix.