I have a character rig and mesh that seems to have been working fine, but in the current scene, something is different. The hand controllers are the easiest to spot - I have a weapon object with a Child Of constraint to the hand.ikR controller, and as I move the hand controller in Pose mode, the hand mesh gets offset from the weapon. Mesh seems parented to the Rig properly, and other character mesh/rig sets in the same scene are working properly. What's going on?
It took a little bit to track down the problem, and it turned out to be a pretty simple issue. The character Rig object, in object mode, reported its Scale to be other than 1. Worse, X/Y values were different than the Z value.
So if this is happening to one of your rig/mesh sets: before you go thru the weight painting, editing the mesh, digging deep into the rig structure... just drop back into Object mode, and check the Rig object's scale. If it is anything other than 1, that might be your issue. It looks to me like a rookie mistake, but if there's any other rookies out there running into this, perhaps I can save you some time.