Crimson Shelf and Warped Shelf can be used as fuel for the Furnace.

Steps to reproduce:

1. Place items that can be baked in the slots at the top of the Furnace.

2. Place a Crimson Shelf or Warped Shelf in the slot at the bottom of the Furnace.

Expected Result:

Crimson Shelf and Warped Shelf are made from Nether wood, so they cannot be used as fuel for furnaces.

Actual Result:

Crimson shelf and warped shelf can be used as fuel in furnaces, just like other wooden shelves.