Creative+Survival

If a hopper has any item 'A' resting on top of it (uncollected) due to all 5 slots containing some other item type B, then item B can't be collected either until item A is removed from the top of it- even if there was sufficient room for item B.

To recreate: fill the 5 hopper slots with 1 dirt each, then perch a cobblestone item on top of the hopper. Then throw dirt at it and most or all of the item entities will bounce away or simply rest on top of it as well until the cobblestone is removed.

I'm pretty sure this is a primary reason why peoples' item sorters seem to give them so much odd behavior lately and I have not seen this reported anywhere before.