I had attempted to use the engraving anvil on a suit of chainmail, and give it acid protecting properties. It said it was done, I took it off the anvil and saw it had no property. I tried to put it back on and replace it with concentration +2. It said the suit was already modified.
The character I did this on was William Kingsmouth.
As an off note, would it be possible to get the 500gp back to re-make the suit and make it engravable?
Find me in IRC to get reimbursement. You didn't mention the cost of engraving was 0 though, which should make it clear something was amiss. I've fixed it.