The idea of "confirmation bias", which is the tendency to interpret new evidence as confirmation of one's existing beliefs or theories, has been covered before in the abstract in this blog as being a route to erroneous judgment and mistakes on the farm.
Let's take an actual story to see what this looks like in the field.
What happens is the grower finds a suspect pest arthropod on his plants, and calls the person serving as the Pest Control Advisor to check it out. There is no visible damage in the field. The guy comes out, takes a look and says since this pest is quite small and has a lot of benign kin, tells the grower that it's going to take a trip to headquarters and some time to get a positive identification on this deal.
Lo and behold a little while later, the grower calls back and has indeed found damage that matches that of the purported pest. He goes ahead with the spray, which probably is the conservative thing to do, but will certainly disrupt the biological controls already in place in the field and cause problems further on down the road.
WHOA. Did you see what happened here? The grower was forcing his own belief, that the arthropod in question was a pest, on the situation in the field, and to support that belief, was discovering evidence, the damaged plants, to support this preconception. In other words, once the possibility of a certain pest was brought up, he found evidence to support that belief, even though a clear positive identification was yet in the offing.
I can't emphasize how important it is be alert to the snare of confirming one's own bias. While it's only natural for us to build up stories and find evidence for things we want to be true, if we aren't we aren't careful about managing these tendencies we can make very costly mistakes, both in terms of money and time.