If you go to input items into a chest mine cart when there is more than one mine cart occupying the space, (two mine carts pushed halfway over a rail, so the hopper is pointing in between them), the hopper only recognizes the first hopper to enter the block-space as viable storage, and will not place or remove items from the second until the first mine cart exists the block below the hopper. This problem also occurs when mine carts turn corners and if they aren't on a rail, this also occurs. This problem also applies to hopper mine carts.