If you create a Dashboard and share it with Group A, the memebers of that group will have access to all of the cards and datasets on the dashboard for any drills down cards to different datasets that might exist at that time. You can then update the drill down of a card on that Dashboard creating a card that is linked to a completely new different dataset. At this point Group A will not have access to that drill card.
When a user of Group A browses to that drill down they will receive an error "An issue has occured during processing. We are unable to complete the request at this time." and the data will not be displayed.
You can then Share that same Dashboard to Group B, and they will then have access to all of the cards and drill downs including the ones that Group A do not have access to.
So despite both groups being granted access to the same Dashboard, they will have different experiences based on the order of events of card creation and dashboard sharing.