Edit
The post's thumbnail image is missing from the server or corrupted.
Note that a thumbnail is not necessarily "missing" just because it is blank or odd looking:
- transparent backgrounds for very old posts display as black, sometimes resulting in solid black thumbnails.
- The first frames of animated gifs or videos may be solid white, solid black, or may be a thumbnail surprise.
These images can usually be fixed by requesting a replacement in topic #16765.