According to Ian Stewart's 2013 Symmetry: A Very Short Introduction (pp. 119-120),
Experiment and theory suggest that superposed states should not be observable as such; only individual eigenfunctions can be observed. More precisely, observing a superposition is delicate and only possible in unusual circumstances; until recently it was believed to be impossible. [My emphasis.]
1) What experimental and (or?) theoretical breakthrough (presumably) is he referring to in the second sentence?
I initially guessed that he refers to the "Quantum Microphone", but I'm not so sure. However, if he is, then 2) how does that count as "observing a superposition" and how wasn't that theoretically expected? ("Yup, quantum mechanics still works.")
Edit. I'm not sure whether it helps, but I'll quote the continuation (and add two tags).
Associated with this suggestion is the Copenhagen interpretation, in which any observation somehow 'collapses' the state to an eigenfunction. This proposal led to quasi-philosophical ideas such as Schrödinger's cat and the many-worlds interpretation of quantum mechanics. All we need here, however, is the underlying mathematics, which tells us that observable states correspond to irreducible representations of the symmetry group of the equation. In particle physics, observable states are particles. So symmetry groups and their representations are a basic feature of particle physics.