I've found a bug with the Stack Exchange image uploader, that occurs when attempting to use a custom URL to link to an image.
Let's take this lovely image as an example; if I try to use the custom URL the first time, I get this error:
However, if I close the image custom URL error message & try again with the custom URL, it works fine:
I have found this bug exists on both Meta & the main Stack Overflow site. I am using Chrome 45.0.2454.101 m, on Windows 8.1 Professional.