Consider the following example:
http://client-gallery.imgix.net/test/0001 Sample.jpg

Source tools from the Imgix's webapp (as well as imgix-core-js) generates the following URL:
http://client-gallery.imgix.net/test/0001 Sample.jpg?s=39a9dc8b6b77986e5208da574ab5f8f2
... which returns Unauthorized response