Joe,
Never encountered this message either. I've googled it up but it yielded no additional clues, except for the notion that the message seems to comes from a Javascript error on the upload page.
I reckon that simply reloading the page and re-uploading does not make the error go away? And, have you ever encountred it with another image file?
Lastly, which browser are you using?
Thanks for taking the time to report this.