Quite simply, make a box out of containers, put any kind of entity into it. Surround the barriers with TNT, ignite them. The entity inside will take damage and eventually die. If any drops are left behind, the player is able to pick them up just by walking right against the barrier. Creepers are also capable of hurting players on the other side of a barrier.

I expected barriers to provide a shield from all influences on either side of them.