Could observer effect cause electrons in one orbital to be found in another?
Your intuition is exactly right. Let's consider an atom of neon. It has ten electrons, which we describe as being in 1s, 2s and 2p orbitals. But the electrons are indistinguishable from each other, and accurate wave functions allow all of them to be in all orbitals. Only when we make a measurement (or some other sort of &quot;observation&quot;) do the electrons behave as if they are localized to individual orbitals.
For example, if we measure the binding energy of the electrons by photoelectron spectroscopy, the result will always be an eigenvalue of the energy operator (aka Hamiltonian). But that does not mean that any one electron had exactly that energy prior to the measurement. As is always the case in quantum mechanics, systems that have multiple eigenstates exist as a superposition of those states until the measurement, the result of which is always one of the possible eigenvalues rather than, for example, a weighted average of them. So we will measure binding energies consistent with 2p orbitals, 2s orbitals and 1s orbitals with an intensity ratio of 3:1:1.