This is an item duplication glitch that involves the new comparator measuring item frame feature in snapshot 14w18.

How to Replicate This Bug:
1.) Find the east/west edge of a chunk. (Preferably using F3)
2.) Place any redstone current-carrying block on the west side of the chunk edge.
3.) Place an item frame on the block, facing east.
4.) Place a comparator next the block, pointing the opposite direction that the item frame is facing, so that the comparator is in a different chunk than the item frame.
5.) Put any block you want to duplicate into the item frame.
6.) Quit and re-join the world. You will now have a stack of item frames and the item you put in the frame.