This usually happen if in the distribution of the mesh groups there are two (or more) mesh groups that "touch" each other, but the corresponding bones do not.
This happens a lot when using the auto group tool, and it happens with chest and rCollar, lCollar. Or with hip/abdomen with lThigh and rThigh.
Or, the cr2 is missing weld groups. This is rare when rigging via the setup room.

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