Yeah it is a common misconception. There is also a minutephysics video on it (https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs) that portrays the effec

Yeah it is a common misconception. There is also a minutephysics video on it (https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs) that portrays the effect of polarising filters in a misleading/wrong way, leading to further confusion. Polarisation is used in many books and courses as a familiar classical system, which we then use to springboard into Stern-Gerlach, which actually is an inherently quantum effect. Polarisation already appears when writing down the full solution to the classical EM wave equation. On the other hand, the similar effect seen in the Stern-Gerlach experiment with the angular momentum measurements is not: the non-commutivity of the angular momentum operator for different measurement axes is a purely quantum phenomenon.