When you have a Fence and above air and above the air a wood block
Top = Wood
Middle = Air
Bottom = Fence
When you click on the Wood block to place a torch on the fence your launcher will crash.
So if you see picture 2017-04-22_14.54.00.png And when i right click there with a torch in my hand the launcher will crash. and i have to relog.
BTW = i was playing on a snapshot vanilla server.