Zombie Villagers that came from born villagers are unable to ride minecarts.

Here's how you get it:

1. Breed 2 villagers and get only the baby.
2. Let the baby grow into an adult.
3. Have it zombified.
4. Push a minecart to it. The zombie villager won't ride at all. For some reason, zombie villagers from spawn eggs are able to ride minecarts just fine.