Explain the Liskov Substitution Principle (LSP). What happens if it's violated? (Square/Rectangle problem)

LSP states that objects of a subtype should be usable wherever objects of the base type are expected without violating the correctness assumptions of the client. The classic Square/Rectangle example demonstrates the problem: mathematically a square is a rectangle, but if Rectangle exposes independently mutable width and height, a Square cannot honor that behavioral contract. The issue is not inheritance syntax; it is incompatible behavioral expectations.