If you place a chest next to a trapped chest and connect a hopper to one of them, it will react as if it was connected to a double chest (it fills both).

Note that this behaviour is somewhat direction and/or position dependent. So to safely reproduce the issue, place 4 chests and 2 hoppers (as in the picture). Then at least one of the hoppers will put its items in the wrong chest.

edit: this also applies to hoppers sucking items out of chests (second image)