The document discusses N-tier application design, which involves breaking an application into well-defined regions or tiers, typically including a user interface (UI) tier, service tier, and data tier. Common mistakes in N-tier design include having the UI tier directly coupled to the data tier or duplicating validation logic across tiers. N-tiering addresses these issues by enforcing clear separation between tiers, encapsulating business rules in the service tier, and allowing new UIs to be developed without impacting existing applications.