A mispronunciation, folk etymology, and quack etymology of duct tape. The story that duct tape was originally duck tape is an urban legend. No researcher has ever found a single instance of the adhesive tape developed in 1942 being called "duck tape" until the 70s and 80s, long after it was established that it was called "duct tape" and people were simply mispronouncing it. This urban legend has picked up a lot of steam for various reasons, but is wholly unsubstantiated.
People like to tell the story that "duck tape" isn't really a mispronunciation, but is actually the origin of "duct tape," but the story is just that: a story.