When dispensing a minecart in a downwards direction it will float 3 pixels above the ground (minecart texture pixels). This will reset itself upon reloading the world.

Pic 1: Looking straight forward from inside floating minecart.

Pic 2: Looking straight forward from inside normal minecart.

Pic 3: How to re-create

Pic 4: The one furthest away is the floating one.