Bone on one side distorts the other side of the mesh I've been having some problems with deformities whilst posing the limbs on my T. rex model. The problem occurs on the thigh and all of the arm bones.
Not posed
Posed thigh
Whenever I rotate the left thigh bone it causes areas of the mesh on the right side of the model to distort and follow the bone.
Leg angled to side
Rotating left arm joint distorts right
This happens with the arms and clavicle bones. The bones are mirrored and both thigh bones are parented to the pelvis bone in between and slightly to the front.
Any help would be greatly appreciated as this problem has troubled us for weeks.
This issue likely is occurring because vertices on one side have been assigned to bones on the opposite side.

When using a mirror modifier, this issue can be easily rectified by simply un-assigning all vertices from the right side—this can be done manually through the Vertex Groups area in the object data panel (just select all vertices in edit mode and click "remove" for all bone-related vertex groups whose name ends in ".r")

Another option (when using the mirror modifier) is to go through each of the right-side bones and subtract the weight from the left side. (However, when using a mirror modifier this will NOT work by selecting the left-side bones and removing weight from the right.)

If the mirror modifier is not in use, just do the second process on both sides—i.e. remove from the right bones vertices on the left, and remove from the left bones vertices on the right.