Here's another one to add to your list:
The people who assume that they will be decorative -- i.e. won't poop where
they "shouldn't" and won't dig rugs or get into furniture.
 
Many of our adoptees were first with people who have short fuses and who
hurt them so they needed rehabbing.  (Or in Meeteetse's case just needed
some dental repair work to correct the problem that was causing her to chew
on fingers -- and led to her broken tail from those jerks)
 
At an extreme are the animal collectors who get shut down and result in
shelters being flooded with animals, but that's way out there.
 
It would be interesting to know which are the most common reasons.  I'm
betting that not saving for medical care, and moving to somewhere which
doesn't allow ferrets (whether apartment, dorm room, city, state, etc.)
will head the list, but I could be wrong.
 
I think it's a smart idea that you are compiling this list in a cohesive
form.  Will you post the full summary to the FML when done?
[Posted in FML issue 3067]