If the user performs an action with potentially destructive or irreversible consequences, or it is likely that the action was performed by mistake, the system should display a dialog box explaining the implications and offering alternative courses of action, including cancelling the action.
Illustration

The elements of a dialog box
Categories General Layout Guidelines Confirmation Prompts After Returns Confirming Actions Confirming Actions After Previous Message Choosing Between Two Related Alternatives With or Without Previous Message Technical Notes Cross-References