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The error message "FormatException: Invalid double" means that there is an error in the format of the input or string that is being converted to a double data type. This error occurs when the code attempts to convert a string into a double data type, but the string contains characters that are not allowed or cannot be converted to a double. This error can occur due to invalid or incorrect input from the user or a bug in the program's logic. It is an unhandled exception because the program did not anticipate or catch the error, resulting in the program crashing or producing incorrect results.