Sorry, I searched and didn't find that. I searched for "F11 exit fullscreen"
The reason for this (I think) is that hoppers in 13w01a were having problems where items that landed on the rim of the hopper weren't getting picked up. In 13w01b, to fix this bug, they increased the item detection range of the hopper. The hopper is probably picking up the item while the item's still in the cobweb because of this detection range increase.
That's because the hopper below is pulling items out of the inventory of whatever's above it (in this case the hopper above it), not because the hopper above is putting items into the hopper below. To me, this seems more like intended behavior than a bug, and I believe they intend to modify the hopper system so players have more control over when a hopper takes items and when it gives items, and that's part of the reason that the hopper is marked a 'work in progress.'
This is a duplicate of MC-31433