Summary: When you have a dynamic bone applied to your hip bone (but have excluded spine/legs/etc) for something like a skirt, it breaks your viewpoint when sitting.

Repro: Take an avatar with non-humanoid bones attached to the hip. Many MMDs have skirt bones attached to the hip. Apply a Dynamic bone with Hip as the root. Exclude Spine/Left Leg/Right Leg. Attempt to sit in a chair.

Expected results: Player viewpoint while sitting remains where the VRC_AvatarDescriptor was defined (in the traditional case, at the avatar's eyes).

Actual results: Player viewpoint will be about 1 meter above your character's head, far above the avatar's eyes. A good way to look at the top of your own head.

Workaround: You will have to duplicate the hip bone, parent it to the real hip bone, and then parent the bones you want dynamic'd to your "dummy" hip bone. Apply the Dynamic bone component to this dummy bone and set it as root as normal.

This issue appeared in Unity 5.6.