On Safari 8.1 Mac 10.11.
Open a page with an inline image such as google.com. Drag the Google logo to your desktop. It's saved as the proper file name, "logo11w.png".
Now on that same page open the inspector resource tab, open the images sidebar folder and clicked the "logo11w.png" image which will be previewed to the right. Click and drag that google logo image on the right to to your desktop. Rather than being saved as "logo11w.png" it's saved as "Unknown.png." Also if you do the same with a .jpg file such as Apple.jpg it will save it as "Unknown.jpeg" That is, it will change the name to Unknown as change the extension from jpg to jpeg. Also you are unable to drag the icon of the image in the sidebar to the desktop at all.
In Chrome you can drag from either location but when dragging the Google logo image preview from the right as mentioned above it saves it as a .png with 100s of random letters (instead of unknown).
It appears this bug was fixed at some point as no longer happening in Safari Preview.
*** This bug has been marked as a duplicate of bug 141515 ***