Oak Planks in Furnace Bug

I was mining on this server I was on, came back and wanted to see what my friend had in his furnace. In it, was oak wood being cooked by birch to get stone. So I tried this with just coal and it still made stone. (shown in the picture) He said he didn't know why it did that, he said he just wanted to test it. I then decided to take a screenshot and report it to you guys so you can fix it for the next update. (hopefully 1.7 )