Was flying my IFR long cross country yesterday to KBWI when I came under the influence of an illusion. My CFII and I were in actual, when I felt the strong sensation that I was in a turn to the left, even though the instruments told me that I was straight n' level. Having studied the health subjects in the books, I realized I was under the influence of an illusion. I trusted my instruments, ignored the feeling that I was turning, and made it home, duh!. Made me realize why studying even the little stuff could make a difference on a flight.
How big of an occurance to illusions happen to the pros? I have about 240 hrs of flight time, and this was the first time I experienced this, and that I could rely on my training. It was one of the worst feelings I've ever felt. My CFII told me that this is why a lot of people get killed, they trust their feelings more than the instruments.

Those illusions are very real and powerful. It happens regularly in non-VMC conditions, although experienced pros know it for what it is and don't get excited.

Inexperienced pilots can feel disoriented or scared when it happens. Non-instrument rated pilots often die, even though they all had it explained to them in training. It's very hard to overide your instinctive feelings.