You may find that your changes were not saved after reopening a project you were working on earlier. Some of the clips you were working on may be found in the iMovie Trash.
This happens if you closed the document and then clicked Save when iMovie HD prompted you to save changes.
To avoid this issue, save your iMovie project (File > Save Project or Command-S) before closing the iMovie window.
This issue does not affect Mac OS X 10.4 or later.