The fake KAOS ray gun.
Twice in four years CONTROL encounters a scientist who claims to be on the run from KAOS after discovering the secret of invisibility.
Dr. Haskell's invisibility ray gun turns out to be a hoax [Episode #5: "Now You See Him, Now You Don't"] but Dr. Canyon's invisibility spray is the real deal and Maxwell Smart himself is able to make use of it to sneak into KAOS headquarters to retrieve some stolen papers [Episode #100: "One Nation Invisible"].
[Invisibility is also the theme of Novel #2: "Sorry, Chief..."].