A hopper minecart should be able to suck in items from any spot on the block above the rail, but items on the north or west edge of such a block remain unaffected by a hopper minecart below.

Steps to Reproduce:
1. Place a rail, place a full-cube block over that rail, and place a hopper minecart on the rail.
2. Place full blocks on all four sides of the air block above the full cube from step 1.
3. Place two more blocks on each of those blocks to make the next step a bit easier and more reliable.
4. Stand on the north edge of the south block, target the opposite wall of the little pit in front of you, and drop an item (Q by default).
5. Repeat (4) for the three remaining sides.

This behavior can be found as far back as 1.5.