How to reproduce
Get a fire aspect sword
/give @p diamond_sword 1 0 {ench:[{id:20s,lvl:1s}]}
Spawn some rabbits
Kill some rabbits
This message will appear in the console:
Couldn't smelt 0xtile.air@0 because there is no smelting recipe
Seems to occur if a smeltable item isn't randomly dropped that time the rabbit is killed.
Does this also affect other smelts (custom loot tables for example, or beef, porkchop, etc.)?