Version: 2.0.3.1
It appears on singleplayer, and the block is useless until the error dissapears.
The error dissapears on chunk reload.
May be a server-side error, because i tried to do a 'testforblock anvil' and didn't match.

It stopped happening, probably because some mods updated.
Also probly was fault of mods like texfix.

Thanks for information. For future reference, check back on your created issues and close them yourself when they are fixed.

I just noticed that the anvil worked about an hour ago.

Please provide me with your latest debug.log file and a complete mod list.