I've found a bug, an extra hand appears above the screen. Doesn't look right. It happens as engineer. (sorry for the large screenshot)
So instead of using the hands that are using the weapon, you need a third hand clock back the weapon. I am not a fan of this bug because his hand covers a good portion of his screen.
It's not a bug. It was a commonly used technique back when animation tools were far less developed (i.e. circa 2005) to do some motions on the weapon. Other games of the period have had 4/5 hands. It's only really visible at aspect ratios that are kind of higher than normal. At 16:9/10 you can just about see it at the corner (the tip of the finger) if you spin around fast, but it's pretty much invisible. Obviously at abnormal aspect ratios, things start fisheyeing a bit and things like this can be seen. Honestly, you start using aspect ratios like this in any game, things will become a little weird. You might see the main 2 hands floating and shit. I remember seeing that in Borderlands for certain.