A small change to the interface for extremely wide (or tall) cropped images showed up in a recent update. I think this was in 3.4, but I could be wrong — I didn’t spot it right away.
If you crop an image extremely wide, like this:
The thumbnail in the Browser will now look like this:
So if you spot this oddly jagged, torn-edge looking thumbnail in your Library now, you’ll know what it is. No you can’t see the entire image in the thumbnail anymore, however I think this is far better than seeing a wide, 5-pixel tall strip of… wait what is that dark smudge, anyway?