So I have this bear mesh I made. Set up the armature and weights. Now I'm realizing that if I move the bones in certain ways these holes open up. How do I close these up? I don't know if it's related but I used a mirror modifier.
In your particular example, when the mirror modifier is not yet applied you have to enable Clipping in mirror modifier, select middle edge loop and move it towards the center of the mesh on Y axis until you won't be able to move it anymore.It will fix the problem.