Preconceptions lead to "selective" attention.
It is well known that preconceptions are filters for perceptions, that also can be interpreted as preconceptions being "selectors" for attention and that gets really clear when we look at some details in classical Ufology.
In Jacques Vallee book "Forbidden Science", there is an account where Vallée recount a letter that he received from Aime Michel, in page 18, where he said, quoting:
"My book gives a misleading feeling of simplicity because of the restrictions I imposed on myself. There are some sightings, as credible as those I have quoted, where the witnesses saw the object disappear instantaneously without any spatial displacement. There are others where a solid geometric object changed shape in a fraction of a second. Can you imagine a pyramid turning into a cube?
Remember what Poincaré said about the fundamental importance of solid bodies in our logic."
We see here a reference to polymorphism/morphing that is almost completely ignored in standard ufology accounts.
Now in Jonh A. Keel book "Operation Trojan Horse", he made an analysis of sighting reports and found that the number of "anomalous" objects was really bigger that the reported "saucer" shaped or "mechanical" looking. Again preconceptions made the "fanatical "nuts and bolts" ET believers", using a Keel's phrase, to completely disregard the facts.
The clear expression of that "selective" attention was given by Stanton T. Friedman:
"Flying saucers are, by definition, unidentified flying objects, but very few unidentified flying objects are flying saucers. I am interested in the latter, not the former."
That "selective" attention is ultimately unscientific and had been one of the reasons for the stagnation that classical Ufology had been for many years.