Sometimes you’ll find interesting explanations about why specific usages are problematic. This one caught my eye recently. It’s from the entry for duct tape in Bryan Garner’s Dictionary of Modern American Usage. Garner quotes a newspaper articles to explain why people spell it duck tape (or ducktape).
duct tape. So spelled—not duck tape. One writer explains the origin of the misuse: “Duct tape is so seldom used on heating ducts that most people mispronounce it ‘duck tape’ as they use it to mend parkas and sheath short cables. I came close to using it on a duct once . . . .” Ed Quillen, “It Even Works for Hanging Pipes,” Denver Post, 20 Feb. 1994, at E3. And the misuse is common […] In the early 1990s, one company—Manco, Inc.— developed the brand name Duck Tape for its duct tape; the trademark, if the product is successful, will only aggravate the confusion.
This explanation that the spelling is lost because the tape isn’t used to repair ducts is fairly ridiculous. The argument is that users don’t’ associate the tape with ducts anymore, because it’s used for mending parkas and sheathing cables. But how on earth does that lead to an association with small waterfowl? Has anyone ever used this product to tape up ducks? And if they have, were these people ever reported to any kind of animal protection organisation?
There is a simpler and more plausible explanation for the ‘erroneous’ spelling of duct tape as duck tape. It is a linguistic one, based on pronunciation. Since in casual or rapid speech it is difficult and awkward to pronounce the two consecutive ts in duct tape separately, they will merge into one. The pronunciation becomes ‘ductape’, which in spelling has been reanalysed as duck tape. There is also another possible explanation that says that the original name was duck tape because it was based on a canvas (also called duck cloth) that was covered with a rubber adhesive to make a waterproof tape. So either it’s a case of reanalysis or of two different but co-existing names for the same thing.
P.S. The product ‘Ducktape’ has become very successful. A Google image search for duck tape reveals that it comes in many colours & designs. It even turns up if you search for duct tape!
Bridging the Unbridgeable 
Actually, duck tape is apparently the original form. It was developed as a waterproof (hence duck-like) tape. The OED has duck tape dated to 1899 and duct tape to 1965. It says that duct may be an alteration of the earlier duck.
That makes sense. The OED doesn’t give the etymology for duck, but it’s said to come from Dutch doek, meaning cloth. I still think, though, that the ‘confusion’ (= variation) between duck tape and duct tape stems originated in pronunciation, although perhaps in the opposite direction as it were, from the one I mentioned.
Agreed. Perhaps the fact that duct tape isn’t used on ducts is a confounding factor, but I think at its root it’s a phonetic problem.