What is “Image corrupt or truncated: <unknown>” and what does it mean?

This can often be seen while using Firebug in Firefox, but it actually has nothing to do with either and is just the error being reprinted in in firebug in most cases.

It comes from an image that has had it’s extension changed from say .gif to .png or .png to .ico. Eventhough .png and .ico file types are very similar, they are not the same, so an error occurs.

This will occur quite often when a website’s favicon was originally a .png and got changed to a .ico.

The image itself will in most cases still open and be visible in browsers etc but should be re-saved using an imaging program.