Large red mushrooms give you 3 unobtainable blocks when using middle click, a block for the top center of the mushroom, a block for the side of the mushroom and a block for the stem of the mushroom. (See photo attached)

Grey mushrooms give you 2 unobtainable blocks when using middle click, a block for the top of the mushroom and a block for the side of the mushroom. (See additional photo)

The correct block types are in the creative inventory, you should only be able to obtain these blocks. (See additional photo)

Steps to reproduce

• Use mushroom biome seed 106854229
• Middle click a large mushroom.
• Now you should have an unobtainable block in your hot bar.

OR

• Be in creatuve (cheats must be enabled)
• Set time in the world to Night.
• Plant a red or gray mushroom
• Bone meal the mushroom.
• It should now grow into a giant mushroom.
• Middle click the edge of the giant mushroom and you will get unobtainable blocks that are not in your inventory.