There is a large plothole in "Scoobra Kadoobra" from The 13 Ghosts of Scooby-Doo. To break the Sleep of the Centuries curse which Maldor the Malevolent placed on Daphne, she needs to be kissed by a great Danish prince. At the end of the episode, it is revealed that because Scooby is a Great Dane, he could kiss Daphne and break her curse. However, Scrappy is also a Great Dane, and thus should have been able to break the Sleep of the Centuries curse.
A possible theory to fix this odd continuity error is that Scrappy's father could have been another type of dog (we already know Scooby's sister, Ruby, is a Great Dane), and a purebred Great Dane was necessary for this curse.