Here are some 20th century quotes from eminent scientists regarding its extreme counter-intuitiveness :
• “Those who are not shocked when they first come across quantum theory cannot possibly have understood it.” Niels Bohr.
• “If you are not completely confused by quantum mechanics, you do not understand it.” John Wheeler.
• “If [quantum theory] is correct, it signifies the end of physics as a science.” Albert Einstein.
• “Quantum mechanics makes absolutely no sense.” Roger Penrose.
• “It is safe to say that nobody understands quantum mechanics.” Richard Feynman.
• “I do not like [quantum mechanics], and I am sorry I ever had anything to do with it.” Erwin Schrödinger.
Such problems continue in the 21st century - see, for example, this 2016 paper in Philosophical Transactions of the Royal Society A which outlines some Master Quantum Oddities :
“A promising candidate for a ‘master quantum oddity’ is the unique status of the non-event ,i.e. a ‘counterfactual’. This is an event which could have happened but did not.”
The authors, from the The Israeli Institute for Advanced Research, and the H.H. Wills Physics Laboratory, University of Bristol, UK, cite the example of the ‘clicks’ from Geiger counters as they register nuclear decay events – but are the gaps between the clicks – which they call ‘non-clicks’ just as important?
“Like all quantum interactions, the non-event is formed by the conjunction of forward-plus-backward-evolving wave functions. Then, it is another feature of such dual evolutions, namely the involvement of negative masses and energies, that enables Nature to make some events ‘unhappen’ while leaving causal traces.”
The authors also coin the phase ‘Quantum Oblivion’ (QO)
“ QO thus turns out to be an event which, in some deeper sense, has occurred and then ‘unoccurred’.”
And all this, they say, has implications for several foundational issues in classical physics, :
“ bearing in mind that quantum non-events far outnumber events in our Universe.”
See: ‘1−1=Counterfactual: on the potency and significance of quantum non-events’ in Phil. Trans. R. Soc. A 374 :20150242.