I've read that once a measurement prepares a quantum state, subsequent measurements confirm it. After all, projecting a basis vector onto itself is an identity.
But if an electron's spin is first prepared in a horizontal magnetic field, and then measured in a vertical magnetic field, then it will either: emit a photon (down state) or not emit a photon (up state), with equal probability. Subsequent measurements in the vertical magnetic field should confirm the first measurement, but if a photon were emitted in the first measurement, then it would not be emitted again because the first emission would cause the electron to switch to the up state. So the first measurement will show down, but every subsequent measurement will show up.
Somewhere I'm confused.