Teen Angst Zoey had the Clavicle bone weight problem. That bone, if weighted to any mesh in the arms rig, caused stretched triangles using some melee weapons (and other custom items). Also, the arms cannot be simply plopped in the arms rig and used without changing the way it's weighted. It mostly shows when someone animates to the extremes. Some animators would use simplified skeletons to animate with. Sometimes these skeletons would lack all the bones the standard Valve biped would use. The arms of the survivors were weighted to use each bone in the Valve biped skeleton. Meaning if you had a mesh that was weighted with every joint in an animation set that didn't use all those joints, the weighted areas would have nowhere to place, so they'd stretch to origin as a fallback to prevent the game from crashing from an index error.