Student versions of taxonomy

I'm grading some lab work where students were analyzing various mammal skeletons for bone homologies (Pharyngula discussed some homologies today too). The students also keyed out all the skeletons to order; here are some of the conclusions they came to:

• Cats are in the same order as either monkeys or armadillos (Primates or Edentata; cats are actually Carnivora)
  
• Rats are in the same order as cats or monkeys (Carnivora or Primates; rats are actually Rodentia)
  
• Rabbits are in the same order as either armadillos or rats (Edentata or Rodentia; rabbits are actually Lagomorpha)
  

Why aren't rabbits rodents? While they're pretty similar overall, one differences is that rabbits have two rows of incisors (two large ones in front with two little ones growing directly behind the large one) while rats only have one row of incisors (two large ones in front).

On a happy note, most of my students figured out the taxonomy with ease. I had to grade >100 papers to come up with these various errors (and many of the problems listed above are understandable based on our key).