The back shifts a little.but only when you are leaning back on it and you move. No matter how much you tighten it down. So you may have to. over time have to tighten down, every so often. And dont panic if theres is 1 extra small bolt. There is no other place for it. I have no clue what its for either. 
Other then that i love my chair. I use it for my gaming and for my drafting table. When im doing my art stuff. The height adjustment is perfect for it. At least for me. Im 5.2  i have the orange chair.