this may only be a bug/exploit on old/slow systems as mine is that most would shrug off as "lag"

in single player this works but have not tested multi-player as i guess as this is an internal server related issue.
if to test in multi-player will need to provide similar environment for the server.
came across this as i was testing to see if unloading a chunk would put out a furnace flame leaving behind unfinished ore to be smelted as research for a mod i am working on.

here was my process:

create generic super-flat world in creative mode.
some place a furnace with lava and wood to make charcoal.
"save and quit game" after retreving first coals from furnace
reload world
see in loading screen "building terrain" for long time - do not think that this should happen. as the terrain should already be built
furnace not there...
reload a few more times consistently seeing the build screen each time unable to to see any player changes made to the world ingame.

i read on the minecraft wiki that complete air sections would not be saved to disk. that may affect improper "building terrain" load screen and finding any work done was erased on the superflat world

so then i deleted the contents of the .minecraftfolder
and had the minecraft launcher force update
and so with a fresh and new .minecraft folder
Create new world
let world load and gennerate for 5-10 min
break or place a block
"save and exit game"
load world
"building terrain"
see world with out edits.
reload world
"building terrain"
but see game was edited.
place furnace and some items there to smelt
reload world
"building terrain"
furnace not there
reload world
if furnace is there it is cooking and all looks good.
wait for the recipie a bit take some items out of the heating cue and the results.
reload
look in furnace
see original amount of stuff in the furnace and the most current setup and inventory with all items you put in there.

so the exploit is to repeat the prosses as many times as possible to get nearly unlimited items you would want.

cheating on an old or slow system such as your grandparents old computer that can barely handle the game.

finding this and unable to tell for myself i now ask the question,
what happens to a lit furnace in an unloaded chunk? my guess is that the furnace retains a progress value of sorts to resume upon chunk load, and yet i do not yet know how accurate that value is/should be.

if my mod does not affect the performance, i will have to see about updating or getting a new system.

the attached files are are some of what minecraft generated in the .minecraft folder and they look to me abnormal as i can not view some of them normally and reading one i see that it complains that memory connection is overburdened. they also generated a .LCK file which were unable to load here because the internet Browser said they held no data.
i saw many of these in the .minecraft folder after doing the experiments.