My mesh deform cage is popping when the character’s arm moves up and down. I have 2 shapekeys, one to correct the shoulder when the arm is up and another for when it is down. Near the middle, however, the values on both jump from 0 to 1 or vise versa. The bone they follow does not jump so I’m not sure what’s causing it… After 20 hours, I have no solution…
Download: https://1drv.ms/f/s!Arpp5Em-H58-hewJ97cylHGP_-6C_g
It might be a weight painting problem 
Be sure to check your mesh cage. Your mesh that is being deformed is poking through your cage. Best way to see it is to put a material on your cage, change it to solid and them put a solid contrasting viewport color on it like deep red. Play the animation and you will then see the mesh poking out of the mesh cage. You can try that.Fix the cage so it doesn’t poke out.
If your driver is reading the raw rotation angles from the bone then those values might pop if it has some rotation on all three axes.
It’s easy to test, like in this GIF I used in this post:
