I was buying Coal Ore today from colebex when I noticed my inventory was full, yet I was still adding money to the trade sign. Usually, there would be a warning telling me I had a full inventory. I did the math and I bought 700 Coal Ore, but in the chest is 650.
It's a recent bug, due to 1.10 or 1.9, I'm not sure. Unfortunately it has not been fixed yet, the only thing I can tell you is to pay attention next time you interact with trade signs, never buy with your inventory full or almost full. Oh, and trade signs are buggy also when you break them (if you own them, of course), so don't store valuable stuff in trade signs for now.
