I first had this happen with ladders. I would have a gun in my hand, either pistol or rifle, and mouse wheel symbol click, not the scroll list option, to climb the ladder. It would do the animation to put the weapon away but sometimes it keep it just my right hand in a strange angle. it would stay like this after the climb and i was running around. If it happens again i will get a screenshot of it.

The attached picture is from when i was sprinting and went to craft a splint, forgetting that it needs to stop you from moving to make one. But this is similar to what happens with the ladder.

I felt like i ran faster like this then with the gun normal in both hands. {F30783}