Two sensors that both turn a load on — by switching the opposite side of the exact same circuit. Confuse them in the field and the load either does nothing, or something worse.
A standard 3-wire DC proximity or photoelectric sensor has a V+ pin, a V-/0V pin, and a signal pin usually labeled OUT. The V+ and 0V pins just power the sensor's internal electronics and stay connected the entire time it's running. The OUT pin is where the difference lives: it's driven by an internal transistor that switches on trigger, and that transistor is either an NPN type or a PNP type. Both accomplish the same end result — energizing a load when the sensor detects a target — but they do it by closing the circuit on opposite sides of that load, and that single fact is the entire reason the two are not drop-in substitutes for each other.