I use furnus on my server and I keep getting this in the console. It also shows purple and black textures in game. After reading the problem it says to register the ore dictionary after the GameRegistry.
This occurs in the latest version of Furnus. 2.1.8

[12:46:30] [Server thread/WARN] [FML]: ****************************************
[12:46:30] [Server thread/WARN] [FML]: * A broken ore dictionary registration with name dustIron has occurred. It adds an item (type: class mrriegel.furnus.item.ItemDust) which is currently unknown to the game registry. This dictionary item can only support a single value when registered with ores like this, and NO I am not going to turn this spam off. Just register your ore dictionary entries after the GameRegistry.
TO USERS: YES this is a BUG in the mod Furnus report it to them!
[12:46:30] [Server thread/WARN] [FML]: * at net.minecraftforge.oredict.OreDictionary.registerOreImpl(OreDictionary.java:679)
[12:46:30] [Server thread/WARN] [FML]: * at net.minecraftforge.oredict.OreDictionary.registerOre(OreDictionary.java:651)
[12:46:30] [Server thread/WARN] [FML]: * at mrriegel.furnus.proxy.CommonProxy.init(CommonProxy.java:28)
[12:46:30] [Server thread/WARN] [FML]: * at mrriegel.furnus.Furnus.init(Furnus.java:31)
[12:46:30] [Server thread/WARN] [FML]: * at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[12:46:30] [Server thread/WARN] [FML]: * at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)...
[12:46:30] [Server thread/WARN] [FML]: ****************************************