I set the sorting mode to manual in the task manager settings. Now when I rearrange taskbar entries (e.g. I drag the Thunderbird entry to the very left), a shortcut is automatically created (by displaying a large application icon) and placed in the panel (in addition to moving the taskbar entry to the desired location). The correct behavior would be to just move the taskbar entry without creating a shortcut icon.
*** This bug has been marked as a duplicate of bug 324391 ***