iMovie 2.1.1 for Mac OS X: Scrambled Thumbnail Images in the Timeline

In some situations, thumbnail images for clips may become garbled or scrambled. This can happen after performing a Split Clip function, followed by an Undo and then a Redo action.
Symptom

Thumbnail images for clips may appear garbled or scrambled.

Solution

The thumbnail is a visual representation of the media contained in your clips. The symptom described above is cosmetic, and does not affect your video content. Apple is investigating possible solutions for this. If you
split the clip again, the first clip will keep the scrambled thumbnail, while the second clip generates a new thumbnail.

To work around this behavior while keeping the same clip length, you can apply an effect such as Brightness/Contrast, with flat settings, so the clip and its thumbnail are re-rendered.
Published Date: Feb 18, 2012