The Crocoduck fallacy is one made by apologist Kirk Cameron and refuted immediately. It is an example of a straw man fallacy. Even though it was refuted, Cameron was found to continually use it, mostly with his partner Ray Comfort (the banana man).
The argument states that for evolution to be proved right, all sets of transitional fossils would need to be completed and, according to Kirk, creatures such as the "Crocoduck" (a duck with a crocodile head) would need to be discovered.
This fallacy doesn't need much explaining: the theory of evolution does not state that a crocodile has to become a duck, or vice-versa, or that such a thing must have happened for evolution to be correct. Even if a duck were supposed to have evolved directly into a crocodile, nothing says that a duck would have a crocodile head aside from the Photoshopped picture at right.
An intermediate form between a duck and a crocodile is not to be expected because birds and crocodiles are only distantly related. Birds evolved out of an animal similar to Xiaotingia and Archaeopteryx while crocodiles evolved out of different types of reptiles called archaeosaurs more than 50 million years earlier, see Crocodile evolution. Intermediate fossil forms between the birds and their ancestors have been found as have intermediate fossils between the crocodiles and their different ancestors.
The Crocoduck fallacy demonstrates the complete lack of understanding of Comfort and Cameron regarding biological evolution. They apparently believe that Darwinian evolution requires a direct, single-step ancestor between every two species on the planet. In other words, there must have been not only a crocoduck, but also a croco-beaver and a beaver-duck predating the crocodile, beaver, and duck.