The bug

Hoppers and minecarts with hoppers can draw from minecarts with chests and minecarts with hoppers if those minecarts are on a sloped minecart track leading up to or down from a solid block directly above the hopper.

In the attached screenshot: I expected the hoppers to only draw from the minecart with chest in the first, second, and fourth designs. Instead, the hoppers in all but the seventh design successfully drew items from the minecart passing above them.

I have tested this with a minecart with hopper in place of the minecart with chest, and with minecarts with hoppers in place of the hoppers. The same behavior occurred in each of these circumstances. Simply running a minecart with chest through a block one above and one to the side of a hopper without going up on top of the block above the hopper does not exhibit this behavior.