What happened

When you place a Minecart with a container (like a locked hopper or chest minecart) below a sideways facing hopper, it fills it items in that minecart.

How to reproduce

1. Place a hopper facing against a wall
2. Place a rail below the hopper
3. Place a minecart with chest on the rail
4. Put an item in the hopper

You will see that the item gets transfered into the minecart.

The bug in detail

When Minecarts are placed in such formation (one block away to the North or West), Hoppers automatically transfer items to those Minecarts regardless of any conditions. MinecartChests and MinecartHoppers act the same.
If you place any items in to the red or orange marked hoppers, items will transfer to the the blue and green Minecarts even if no Minecart is present under the red or orange hoppers.
NOTE: Chests and Hoppers do not act this way - just Minecart versions. This is not consistent and is considered a bug.
comment by qmagnet on this report