Working on an application that was already built before I came to it, I came across this error:
"Image corrupt or truncated: <unknown>"
It only appears in Firebug's console in Firefox (it did not show up in IE or Chrome).
After Googling a bit I found that a lot of people seemed to be blaming it on favicon.ico. As far as the error went though, no line number is indicated and the favicon actually does appear in the Window's title bar/tab.
Thinking that maybe there was something wrong with the format of my favicon.ico, I opened it in Adobe Fireworks to see if it was possibly misformatted in some way and if there was a chance that re-saving it might help. Fireworks told me that it was actually an 8 bit BMP file and I found out through more Googling that ICO files can be made by simply saving your images as a BMP and then changing the file extension to .ico. I repeated this action but still wound up with the same error.
Out of curiosity, I browsed directly to the favicon and noticed that in the window title/tab title the following appeared: "favicon.ico (BMP Image, 20x20 pixels)". So it was actually still being interpreted by Firefox as a BMP image, not an icon.
Confounded I decided that even though this BMP/ICO file extension conversion seemed simple enough, Firefox must be interpreting it differently still. So, resorting to an online ICO file generator () I managed to fix my problem. I uploaded the BMP version of my icon (20x20 pixels) and generated an ICO version at a more official icon size of 16x16 pixels.'
Voila! No more error and the title in the window/tab now appears as "favicon.ico (ICON Image, 16x16 pixels)" when you browse directly to the favicon.
PS. Even though browsers these days seem quite capable of picking up a favicon in the root folder of your application, it's always a good idea to explicitly point to it:
<link href="http://[YOURWEBSITE]/favicon.ico" rel="shortcut icon" type="image/x-icon" />