Information

[touqra]

Quantum evolution is said to be unitary which was seen as an information preserving process. But, isn't it that quantum phenomena do not depend on previous experiment, I mean, eg, Stern-Gerlach experiment. If I put three filters, ZXZ, where z represents Z axis spin filter and X represents x axis spin filter, the outcome of the experiment is not dependent on the first Z filter?
Did I get anything wrong?