If you use a handheld weapon/tool and then quickly take off your helmet, that handheld device will get stuck to your hand.
I can't get it to happen when putting your helmet on though, so it's only 1 way.
Basically the animation to take of your helmet doesn't wait till the animation to holster your handheld weapon is finished, this causes the object to not despawn from your hand (since it never reaches the end of its animation).
Afterwards the aiming animation doesn't work anymore either.