Hopper minecarts will not pick up items above them which are in a stack of 10 or more quantity through half slabs. Quantities with 9 or less are picked up fine.

Steps to reproduce:
Place a rail with a half slab above it.
Place a hopper minecart on the rail.
Place a group of 9 or less items on the slab, the cart will pick them up.
Place a group of 10 or more items on the slab, the cart will not pick them up.