I tested it on pigs and sheep and that's what happened. Refer to the image in the attachments.
MC-2530
Breaking a minecart while a mob is inside it causes the mob to sink into the ground
Resolved
Duplicate
0
Sam Clews
(Unassigned)
Unconfirmed
Minecraft 1.4.2
Retrieved
This ticket has been marked as a duplicate of MC-4925 , in favor of the better description/information provided.