I suppose this can apply as early as that moment when we get the true story behind various holiday-associated supernaturalities and then into our more mature years when previously dear beliefs give way to circumstances. For example, that job we have dreamt of since the age of twelve turns out to be nothing like we imagined; or, our faith beliefs are shattered by some event; or, what we thought about people, in general -- say, the belief that most people are kind at heart -- gets proven (at least for us, personally) wrong.
These are big examples. But the more insidious shift happens on a smaller, more "viral" level. In these cases, we are not talking about a tectonic drift in philosophical belief or of theological understanding, but in a tiny thing that the thinker has never even though to doubt; it has just been a brick in the foundation of his life, down there, under the house of his body and soul, doing its job, silently, even invisibly. A small, but integral part of that which defines his existence.
If circumstances, then, wind up exposing that brick and if a good dusting-off of the brick shows all is not as it seemed, the impact of this is one that is not expected. Maybe the brick gets put back into its place, but it is replaced in the consciousness by the nagging idea that it is cracked. Not enough to bring the house down, but enough to raise questions as to whether the house is what it has always seemed to be.
All of this vague metaphor calls for a real example. Imagine you have a best friend who has, for years, supported you and encouraged your abilities as, say, a real-estate agent, but then you find out that he really believes you are not very good at what you do. It's not the fact that he doesn't think you are good that is the problem; it's the fact that what you thought was a brick in the foundation of your relationship -- maybe for decades -- is not quite what it seemed. If what you saw as mutual respect was an unconscious element of your relationship's definition, now, you are face with the fact that you have been wrong -- maybe for a day, but maybe for decades.
Going into work, for example, we expect that we may have the most awful day ever, and we can be ready to peer over the treetops of the dark forest of our day and say: "There is my home. I'll be there, soon, and this day will be behind me." We are ready to defend against that kind of bad mood. But the new idea we never see coming? Bam. Next thing you know, you're sitting up in the gloom and trying to shake the fuzz out of your head. And until that fuzz is gone, the real reasoning; the real navigation out of the fog cannot begin. It can be really hard to shake off that kind of a fog.