Short description of error
Toggling the mode on the armature causes a COW update that destroys cached B-Bone segment transform data. This data is then not recomputed even if there are users that need it.
Exact steps for others to reproduce the error
- Open the file.
- In the properties panel there is a Copy Transforms constraint. Changing the head/tail value moves it along the B-Bone.
- Tab the armature to Object mode.
- Now changing the head/tail value will snap the constraint away from the B-Bone to its underlying straight shape.
This also happens with Armature modifier, but the constraint case is simpler to show.