Upon building a bamboo farm with a hopper minecart pickup system, I realized that because of the teleportation behavior of minecarts traveling at speed, it doesn't pick up blocks above the corner even though the rails cover this area. I'm assuming this is because the entity doesn't technically enter this block, but the rails would lead a player to think they should be picked up. In order to recreate this, I suggest putting a minecart hopper on a powered rail about 8 blocks away from a corner, and then  drop an item on top of a solid block above a 180 corner.