There is an unstable mode in some aircraft (maybe only small aircraft?) where it enters into a kind of spiral dive. When you are sat in a plane where that is happening (I have done this in a training exercise) you really cannot feel anything unusual at first - the forces you experience from the different accelerations cancel out. Until all of a sudden you do feel it - at that point you are pitched with the wings at about 45 degrees and the nose pointing about 45 degrees towards the ground. Suddenly you feel it a lot. That is a very scary experience, and one of the reasons to ignore your body if you are a pilot.

When I experienced it it was in clear skies, so it would be rather obvious that it was happening just by looking out of the window, but apparently this can happen to inexperienced pilots in clouds.

See http://en.wikipedia.org/wiki/Graveyard_spiral

when instruments were first introduced to planes, most people thought that something caused all instruments to malfunction like crazy as soon as they entered clouds due to the ghost spatial effects the pilots assumed were true ("I can't be turning, I feel like I'm going in a straight line!" or "I can't be going straight, I feel like I'm turning").

If I remember correctly, the average lifetime of a non-instrument trained pilot in instrument only conditions is measured in seconds, because these ghosting effects are so strong.