If one isn't quite noticing or accepting what is really going on in the present, but one responds based on one's thoughts or feelings about what ought to be, then one is apt to collide with what is really going on.
The representation of what ought to be is very often stored unconsciously. The person is not consciously aware of what she is expecting. She will simply act or react as if it is reality. And when it isn't she will clash with the actual reality she is operating in to a greater or lesser degree.
Unconscious "oughts" often come out of one's experiences in the past. Past events might have burned an imprint into the sub-conscious mind of "the way things are". Particularly overwhelming, traumatic incidents have a tendency to spill over into other times and situations.
One's hopes for the future might also create ideas of how things "should" be that are different from how they are. One might have wanted to be in a different situation than what one is actually in.
Fixed ideas in general are prefabricated conclusions about how things "are". If one perceives through one's fixed ideas one is attempting to interact with something that isn't really there.
The expectation of what "should" be can exist at varying degrees of unconsciousness. If it is only slightly unconscious the person might immediately notice her mistake and correct her idea of what is going on. If it is more unconscious she is likely to put up more of a fight, insisting that her idea of reality is more real than the reality.
Semantic reactions are cleared by exposing them. By finding out what they are, or where they come from, by finding out what the present reality is, and by comparing the two.