Quantum mechanics has been proven valid to an absolutely astounding accuracy totally unusual for theories generally. Quantum physics seems to be the only theory that is not just linear in its early stages. All subfields in science start out linear, like relativity started with special relativity for example, but they never stay that simple for long. Quantum physics seems to be precisely linear, which is also called "unitarity" in this case. Even black holes have been modeled with linear quantum physics in string theoretical approaches. There are those who claim that gravity will introduce non-linearity into quantum science, but according to today’s consensus, they simply divide into different sorts of crack-pottery. Some great names like Roger Penrose are among them, but as far as the elite of high energy particle physicists and string theoreticians in all the prestigious physics departments around the world are concerned, basically all who doubt linearity in quantum physics are more or less serious cases of crackpots (Stephen Hawking has quite recently caved in to that peer pressure).
Quantum theory as it is presented today holds that all microstates, all configurations of the particles that make up your body and brain and environment, have some finite, non-zero probability of occurring. (This is classically known as Boltzmann freak brains and quantum mechanically discussed also under the heading of quantum fluctuations.) There is even a finite probability for you to tunnel into any possible microstate s at all right now. The probability is extremely small, but if quantum physics is truly linear, all these states are possible, meaning their probability is larger than zero: P(s) > 0. Probability is a frequency in empirical records, a belief that is updated by Bayesian procedures, a degree to which a rational actor expects future. According to today’s quantum theory, even a probability that is much lower than a single occurrence during trillions of lifetimes of the universe subtracts nothing from the internal reality of an improbable state. In some descriptions like chaotic eternal cosmic inflation theory, these states occur an infinity of times  (as if that adds anything to occurring once).
Now I want you to imagine finding yourself in the following situation:
You are holding a bloody chainsaw in your hands having just cut a horrifically painful and fatal injury into your only beloved eight year old daughter in an ultra perverse act of satisfying a disgusting sexual fetish that you never even had before while all cars all over your city are upside down!
NOTE: The upside down cars preempt weak defenses against this scenario via inter-subjective reality: All the people around you clearly see that all cars are upside down, and you could not have overturned them all before you raped an innocent child of your own. Also, do not refuse this out of hand lightly via some 'I do not like thus do not enjoy' "modal error". The point here is physics: You enjoy if a suitable cocktail of molecules (e.g. dopamine) are present, and that presence is a perfectly possible microstate! You find yourself in it - no need for smooth history (QM fluctuations).
I call this the “Terribly Inconsistent Macro-State” S, or short “terrible state”. It is not involving a terrible act in order to shock (say to generate page hits). It is deliberately disgustingly perverted in order to make all you “nice people” out there take a certain position, namely the position that this state S is impossible. You claim not only that this situation is highly unlikely, but that it is somehow inconsistent, say with your memory of that you never had a fetish like that, love children, and would give your own life for your daughter. Yet if the terrible state S is possible, you may find yourself enjoying killing your daughter, perhaps with a missing memory about the ten minutes before, maybe no “strict inconsistency”, but at least inconsistent with what you like to believe could ever be possible.
With dark energy and dark matter and dark terrible states, suggestions of robopocalypses, global suicide and so on, one could conclude that a seriously dark age, indeed the ultimate one is dooming. Well, and why would that be an impossible conclusion? Perhaps just the "God is dead = the end" mistake?
If S is impossible, its probability is strictly zero, P(S) = 0, but again, there is this problem with modern science: The many microstates s of this terrible macro state S, meaning all the many arrangements s of molecules that are consistent with the macrostate S, are all perfectly possible according to what modern science knows! If you believe that quantum mechanics is fine as it is and that it will get no further corrections, you must also believe that there is a parallel world (and those do in general obviously "exist") where you have sex with your daughter while killing her with a chain saw! No, it is not only some sort of unlikely event, but quantum physics as we know it today holds that you will find yourself in that state. Even worse: You are doing it already in a parallel universe next door!
Seriously: Do you honestly believe this?
Most, even most quantum scientists, do not believe this, meaning they believe that this is not true, regardless of how much science supports terrible states. Scientists are not different from other humans: rationalizing believers. Additionally and at the same time, so many look down upon those who venture to suggest corrections to quantum theory! This is one more instance of the usual hypocrisy in established science, where the mass of mediocre scientists often strongly defends a perceived consensus none of them actually fully grasp.
Quantum physics is close to the final answer to "everything". Quantum physics is about the interference of different alternatives of what can be phenomenal – it is not mechanics of a world so much as consistency of phenomena. It is not about small stuff; a radio wave photon can be hundreds of meters long. Quantum physics is our starting to find the description that contains all possibilities. At first we thought this encompasses only the alive and the dead Schrödinger cats in our particular universe, but it includes all the ways a universe may be described to unfold out of its own Schrödinger box, so it contains anyway all possible universes; everything possibly phenomenal for consciousness as such is included. This ultimate fundamentality is precisely why quantum mechanics may stay linear!
However, this does not mean that we already know everything there is to know about what states are phenomenal, about what is actually a situation an observer can be conscious about. It is conceivable and I have defended this against resistance from pseudo-skeptics, that “consistency” (in a wider than merely logical sense) of consciousness plays a role at the foundation of physics where quantum science meets phenomenal actualization (This can be made quite convincing via certain variants of quantum calculations and for example the EPR paradox). The microstate s of the terrible macro-state S may be “possible” according to quantum physics as we know it (P(s) > 0), but terribly inconsistent macro-states S that are based on those s could nevertheless have strictly zero probability (P(S) = 0) in terms of the frequency with which they occur in empirical records of conscious observers.
Research into these questions has the aim of letting the P(S) = 0 hypothesis guide the search for “nonlinearities” that may “cut off” P(s) > 0 if it is too small**. This small probability cut-off and similar suggestions would for the first time in a long while make philosophical considerations guide physics, making philosophy of science at least marginally useful for once. However, I have not found any funding for these questions, neither from physics nor from philosophy.
If quantum physics as we know it today has no corrections from phenomenal consistency, if terribly inconsistent macro-states are possible, is there no more to be said than that their low probability, whatever probability is, makes us label those states as madness? Do we plainly not identify with those of us who tunnel into S, although S remembers having been us? This surely is a philosophical question.
** Similar has been proposed for example via “larger” universes “mangling” smaller ones and of course via gravity cut-offs to small probability. I do not see it necessary to have the cut-off naively physical in a too mechanistic sense.
 A. De Simone, A. H. Guth, A. Linde et al: "Boltzmann brains and the scale-factor cutoff measure of the multiverse" Phys Rev D (2008) http://arxiv.org/abs/0808.3778