This can occur in iMovie 3 or iMovie 4 after you do the following:
- Drag a movie clip from the Clips pane to the Clip Viewer.
- Trim off the beginning of the clip in iMovie monitor.
- Empty the iMovie Trash.
- Quit iMovie.
- Open iMovie and open the same project.
Even though the thumbnail shows the incorrect frame, the movie plays back and exports correctly.
Tip: This symptom does not occur if you don't empty the iMovie Trash.