This is probably topical, and will be far outside the experience of most people.
The average time that a non instrument rated pilot takes to lose control of an aircraft in IMC is 178 seconds. The instruments themselves aren't all that hard to read, and most pilots (before instrument training) think that they can do so....so why all the fuss (and training) for an instrument rating. The difference is that when they are flying in nice weather and looking at the instruments...they aren't in cloud. So, they're actually looking outside most of the time, which has the effect t of constantly resetting the brain/ear.
But, when you actually enter cloud, you can now experience various illusions.
I'm sure there are many here who can describe the design of our inner ears much better than me, but the upshot evolution that did not including flying, is that our ears are subject to a number of illusions. These can be incredibly strong...not just some vague feeling.
Once you take away the visual cues, the ear cannot distinguish any difference between 'down' as defined by gravity, or 'down' as caused by any aircraft movement. And to make it more fun, another part of the ear confuses acceleration with pitch.
This shows itself a number of ways in aircraft. Probably the one that is seen most often is called the 'leans'. In this case the aircraft has slowly rolled away from level, without the pilot noticing. When he looks at the AI, he corrects the error. His ear notices the correction...and even though he's now level, the ear thinks the aircraft has some roll. The feeling fades after a couple of minutes.
The illusion that is possibly topical at the moment is called somatagravic illusion. It can manifest itself in two ways. Basically, you experience a rapid increase in speed and sense that as a pitch up. The reverse also happens. Catapult launches would probably be the extreme case, but it has been implicated in many accidents over the years. Rostov may simply be the most recent.
The most important lesson of instrument flying is that you MUST believe your instruments. Yes, they can go wrong, but that is extremely unlikely, and even then there are workarounds. But, you must never believe your senses....they will be wrong every time.