I searched this problem in forums and didn't find any reasonable answer. Since uploading to R4 this error is popping up in console everytime someone is using the furnace. What to do? Block at 101,57,-2086 is FURNACE but has null. Bukkit will attempt to fix this, but there may be additional damage that we cannot recover.
if its harmless, then ok, i was just kind of worried of the "but there may be additional damage that we cannot recover." part
