Since in the changelog for the snapshots it stated that "Logging out will not cause you to dismount pigs or minecarts upon return", I tried to put a saddled pig into a minecart and then ride it, and when the world was reloaded, the minecart was gone.

Put a saddle on a pig, push it into a minecart, get on the pig, then save and quit and reload the world.

EDIT: Sethbling did a video about the latest snapshot and stacking entities, and this bug is also seen in that video.