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.
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Confirmed in 13w11a.

Is this still a concern in the latest Minecraft version 14w30c? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

This ticket is incomplete without the requested information, no response has been received within a reasonable time and we are assuming the issue has been resolved. If you are still experiencing this issue, we can reopen it at your request.

Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time.

Can confirm for 1.13-pre3.

Confirmed in 1.15.2 and 20w22a.
Affects 1.16 Release Candidate 1
Affects 20w45a
Can I request ownership?