Cross-pattern comparisons worth memorizing

These are the distinctions most likely to be tested, and the ones that actually bite in review:

Confusion	The tell
Bridge vs. State vs. Strategy	Near-identical UML (context → interface, delegate). Bridge: two hierarchies designed up front, both varying. State: the delegate swaps itself and drives transitions. Strategy: interchangeable, mutually-unaware algorithms chosen by the client.
Decorator vs. Proxy vs. Adapter	All wrap one object. Adapter changes the interface. Decorator keeps it and adds behavior, client-controlled and stackable. Proxy keeps it but controls access/lifetime and manages its own subject.
Decorator vs. Composite	Decorator has exactly one child and adds responsibility; Composite has many and aggregates their results.
Facade vs. Mediator	Direction. Facade is one-way and the subsystem doesn't know it exists. Mediator is many-way and colleagues hold references to it.
Mediator vs. Observer	Mediator removes mutual dependencies among a known set of components via a hub. Observer creates dynamic, independently-subscribable one-way links. Often combined: mediator-as-publisher.
Memento vs. Command	Not rivals — partners. Command = "the operation"; Memento = "the state to restore if it's undone."
Facade vs. Abstract Factory	Both hide a subsystem. Abstract Factory hides only creation; Facade hides creation and call sequencing.