The red "missing item" squares when crafting the composter sometimes only appear on half the squares or none at all, confusing new/unexperienced players. 

1. open LAN world
2. gamemode survival (no cheats)
3. grab 2-4 slabs
4. attempt to craft composter by clicking on the composter item in the crafting menu
5. not all red squares will show.

this may not happen consistently but happens frequently