I'm working with a compound clip of a jpeg overlaid with one translucent clip on the left and a different one on the right. I was very careful to zoom in to 300% when using the transform to make sure they appear seamless. (The white line of the transform box is in the way, but enough trial and error, I got it perfectly.) Made compound clip. Now, working with that in my main project timeline, there is an inconsistent white line down the center of the frame, particularly during a Ken Burns zoom out. Viewing at 300%, the overlaps of the layers look sort of all over the place. So I double click on the compound clip to repair it, viewed at 300%, and they're flawless. Zoomed in during main project, it is there. During the pull back it is intermittent - shows up for a few frames, goes away a few frames. I have three sequences like this, and only one is behaving this way.
Much appreciation for your sharing your admirable knowledge!