Since Firefox 4 rolled out and Firebug was updated a new error appeared, even on projects that had previously been error free. Then I started seeing the error:

    Image corrupt or truncated: <unknown>

This would usually only appear on localhost but what was strange was that there was no reference to a file or line that the error was coming from - essentially making it impossible to debug. 

Initially I put this down to the few know issues with Firebug 1.7 and FF4 then later found it started to appear when I added a favicon to a site (with or without the <link> reference). 

Corrupted image

What causes this error is actually a correct response in reading an image that has had it's extension changed from it's original saved format. 

In the same way that changing a jpeg's extension to ".gif" doesn't make it a gif, giving a png a ".ico" extension doesn't make it an icon file. 

Opening the file in an image editor that can recognise Windows Icon files, such as Irfanview, also highlights that the extension is wrong;

 

Renaming the image and then re-saving it as an ICO - Windows Icon file gives you a correctly formatted image and stops Firefox throwing an error.