I opened a furnace with my window on the left side of the screen and saw that weird gui above the normal furnace gui. I might have put the window onto the left side while in the menu, I don't remember. After a few minutes the gui image shrunk from the top down.

The furnace was in the nether cooking netherrack into nether bricks using a lava bucket as fuel. I have added a picture of the bug as an attachment. I have no idea how to reproduce it. If this has already been reported I am sorry, I have no idea how other people would describe this bug. I was also unable to reproduce it :/