Here is something Tarin wrote to me a while back around the time I got stream-entry.
Tarin:
that subtle background of whispers emanates from deep within what is considered 4th jhana territory and, interestingly enough, gets touched upon a bit during the peak of arising and passing territory as well (though usually only briefly and in a way that cannot stabilise). it is from there that many unusual things arise.. among them are: the mundane psychic powers[1]; direct access to any jhana or nana at any time[2]; supramundane experience[3]; opportunities for deep exploration into what appear to be the core underpinnings of personal as well as impersonal consciousness[4]; and, last but not least, huge, and possibly lasting, swells of delusion[5].
[1] such as the ability to understand causality to tremendous degrees, resulting in an ability to 'see' past and future lives, the ability to see and hear, in the mind's eye and ear, things happening at distant locations, and the ability to intuit the feelings and thoughts of others.
[2] rather than having to proceed through them sequentially.
[3] that is, fruition/cessation.
[4] these are the vibratory murmurs from which feelings, and then thoughts, are formed; listening closely results in all sort of weirdness, much of which is not relevant to the matter of resolving fundamental suffering yet may help one develop an appreciation for why resolving fundamental suffering is a good idea, to say the least.
[5] at first glance, the dangers implicit in following seductive commands emanating from a mysterious and clearly powerful source should be obvious, yet they are frequently not, as all too often, by the time one hears the command one is already following it. the dilemma in this territory is that there is no solid distinction between a read-only file and an executable.. here, one ventures beyond the protective and orderly layers of the mind, which millions of years of evolution have constructed so well, into a portal-like territory in which there is virtually no limit to the variety of directions it may suddenly extend.
the lifting of those well-accustomed psychic restrictions can be downright terrifying, which fright can lead the in-built survival instinct we all have to kick into overdrive, causing massive self-aggrandisement, and effecting one's eventual return to physical reality not as the inquisitive explorer one went in as but as a sollipsistic god who has figured it all out (because it is simply too scary to acknowledge the enormity of the unknown that one faced). what results is something similar to people who go on an i-am-god/buddha/all-knowing-consciousness trip during the a&p, but more deeply ingrained and longer-lasting.
Per Yadid's suggestion, I thought of describing the ways in which these kinds of phenomena have affected me, and others might chime in as well. I think it is important to be aware that this kind of thing can happen as a result of meditation (particularly 1, 4 and 5), since being aware of this possibility might make the difference between simply having a rough time with some confusion and uncertainty versus having a full-blown psychotic outbreak, persistent delusion or something along those lines.
The way it seems to work is as follows:
(1) beliefs are things that "feel true," and it is widely known that they have the power to distort perception: what one believes in changes what one sees.
(2) under certain conditions of high-concentration and/or mental power, one has direct read-write access to the area of the brain where beliefs are stored, and it becomes possible to change one's beliefs. This might happen purposefully (mystical traditions such as western magick work with this), or accidentally.
(3) If the process gets out of hand, it might cascade into full-blown delusion. One changes one's belief a little, and suddenly things start to happen that seem to confirm one's belief (concidences, or synchronicities, or visions), causing one to believe even further, in a self-feeding loop that throws one into the realm of paranoia, or prophecy, or the divine, or imaginary galactic heroism involving satan, aliens and energetic entitites (
link), or something completely different, or a mix of the above.
I have never delved deep into this territory, and don't plan to, but I have touched it briefly and superficially on two occasions.
Once during my first Dark Night, I started suspecting that people in my work were commenting negatively about my performance. Then I started feeling that I got "strange looks" when arriving at the office. Then one day a colleague told me he saw me playing saxophone through the window (he lived in the building in front of mine), and I interpreted it as an accusation that I should have been working instead...
Around that time, I had two hallucinatory episodes, one auditory when someone's laughter seemed distorted and diabolical, and one visual, when I saw the face of a guy passing by on his bike turn into the face of a person I used to be romantically attracted to.
Luckily these were so surprising and unusual for me that I decided something really strong was going on, and that the proto-paranoia was part of it, and that I shouldn't be fooled by it, no matter how plausible it seemed. It faded away as the dark night developed into other things (panic attacks became a recurring theme).
The other episode worth mentioning happened during first path A&P, when I became convinced that the universe was an act of love-making between shiva and shakti, and developed my own brand of metaphysics by mixing and matching various things I had read up to that point. I can't recall the details very well, as I recall those times as somewhat similar to a fever. But emptiness was equal to space (
link), and there was something about the point of infinite potential, and they had sex

...
I dunno, more importantly, I remember that I would sit in contemplation of nature and I had the distinct sensation that "everything fit." The imaginations I was spewing in my mind "felt correct," in a way that seemed all-pervading, all-connecting, and "just right." I would have internal monologues that would finish in bombastic sentences that felt true (e.g. "the universe is born of friction"), and then I would nod my head and think "of course!" and "it all makes sense." I would spread the message of love to all those who would care to listen, and pity those that didn't.
Then one day the dark night followed, and the dreamy reverie turned into a nightmare, and nothing made sense anymore, I was scared, etc many of you know the drill. Suffice to say, it was all a big heap of bullshit, and quite a humorous one in retrospect, since it bore no unfortunate consequences (though my family and close friends found me a bit weird for a few months).
----
The basis of all these is belief. It really really (really) feels as though these things "are true" and "make sense". They "sound right," sometimes to the point we downright "know that they are true," and if someone contradicts these thoughts, we may become angry ("who do you think you are?") or complacent ("I will have to leave as the general discussion level on this thread started to deteriorate.").
In a not-very-extreme scale, this kind of thing happens all the time, with everyone, so it isn't very worrisome in itself. It is just about having the persistent self-honesty and critical thinking required not to let it blow out of proportion.