A spectacular error can be anything. Many factors have to come together for an error to be truly spectacular.
This unique 1970-S Proof Quarter from the United States was over-struck on a 1941 Quarter from Canada. This mint error was originally discovered in a group of San Francisco Proof Errors that was auctioned by the State of California. There is a significant amount of detail on both sides showing the design of the Canadian Quarter. This is one of the most fascinating and intriguing proof mint errors ever discovered.