Why is my mesh deforming with a pose?

Question

I am using a model taken from a tutorial to learn how to rig/skin/animate a set of arms with a gun. I created the bones and vertex groups on the left arm first and then symmetrized the right arm, followed up with some touch-ups. The right arm works perfectly when posing however the left arm, specifically the forearm (labelled as Upper_Arm_L) deforms if I move the left hand too much in one direction or another, this does not happen on the right arm. I have gone in and removed all of the vertex groups then reapplied them with no success. I've double checked the bone constraints too with no success. Here is an example of what I mean: Normal pose position, no distortion.

Distortion of the forearm after moving the hand too close.

When rotating the hand, the entire arms come away from the rootbone

As mentioned none of this happens on the other side of the rig.

File:

Answer 1

If you select a vertex of the upper arm you can see that it's assigned to some vertex groups it should not be assigned to:

To fix that, select the upper arm and forearm vertices, go into the Properties panel > Object Data > Vertex Groups and click on Remove from All Groups:

Then select the vertex group each piece should be part of and click on Assign (it looks like you've inverted upper arm and lower arm names so I assign the way it is set currently):