Actual fuel consumption differs from recipe values.

[CynicalBusiness]

For some reason, the fuel consumed when running a factory is not the same number as state in the recipe. The factory will complain it does not have enough fuel, but will finish its cycle anyway.

For example, the Smelt Stone recipe of the Basic Contraption claims to require 10 coal, but actually only consumes 6, leaving the remaining 4 in the furnace.