Affects Version/s: Minecraft 15w38b, Minecraft 15w39a
Fix Version/s: None
Environment:Windows 10 Pro, 64 Bit
Java Version 8 Update 60
When using the Pick Block key (default middle mouse button) on a block that is in your inventory but is not on your hotbar it will cause the item to function incorrectly. This is in survival mode.
If the block moved to an empty hotbar slot when you attempt to place it, it will appear for a second and then vanish. The amount of items in the stack will count down as you attempt this.
If you open your inventory and click on the block, it will return to its original position in your inventory, and its item count will be reset to the correct amount. If you place all of the blocks, they will disappear though the correct amount will return to your inventory once you rejoin the server.
If the block replaced an item on your hotbar, the original item will be placed rather than the one which is currently in your hand. The original item will then return back to the hotbar and - interestingly - will also display another stack in your inventory of the same item. The item that you used Pick Block on will not be in your inventory until you rejoin the server.
Video of this with an empty hotbar slot:
Video of this with a full hotbar slot: