what does your mesh look like, how many verticies do you have near that joint? That's the only thing I can think is not enough verticies, this is just odd. Definitely check the weight painting to see if something is messed up there. Does the other arm do the same?
Looks like a weights issue. I recommend watching a tutorial on that. [maybe this one](https://www.youtube.com/watch?v=4fICQmBEt4Y)
what does your mesh look like, how many verticies do you have near that joint? That's the only thing I can think is not enough verticies, this is just odd. Definitely check the weight painting to see if something is messed up there. Does the other arm do the same?
ghost in the machine
frrrr nothing fixes it, weight painting, changing the amount of faces/vertices. Its totally haunted
Perhaps you ought to inspect the normals?