An error is very hard to correct if we don't suspect there was an error. Often we make an incorrect assumption without realizing we made it, and then draw other false conclusions based on this false assumption. One area where this happens frequently is in finding errors in computer programs. Once I had a very difficult time trying to figure out why a program I had written gave a wrong answer. I would test to see whether certain intermediate steps were giving correct answers and they always were. So why was the final answer coming out wrong? It turned out it wasn't! Earlier I had accidentally run an old version of the program that actually did have an error, thinking it was the current version. If I had realized that my assumption might be wrong about the current version having an error, I could easily have checked it and saved a lot of time.
An assumption that we probably all make sometimes is that what we read in the newspaper or see on television news is true. In the mass suicide by the Heaven's Gate cult, the first report was that all the people were male. It eventually turned out that more than half were female. It probably never occurred to most people at the time (including me) that this fact was in doubt. Fortunately this was not an assumption that had practical repercussions for most of us.
Magicians deliberately make use of people's assumptions to make their tricks impressive. In one such trick a magician asks to see someone's watch, holds it up by the strap, and announces "it now says the correct time, 8:10 PM", and then hands it back to the owner. After doing some deep mental concentration, he announces that he has made the hands move forward. He asks the owner to read the time, which now says 9:30. What happened? Actually the magician spun the hands forward before holding it up and announcing the time. Almost everyone in the audience assumes that the time read out loud by the magician was correct, and so they are mystified because there was no opportunity for the magician to manipulate the watch after the announcement.
Sometimes such tricks are not so harmless. In James Randi's book "The Faith Healers" he describes a trick used by phony faith healers. When someone comes into the auditorium with a limp or other minor problem walking, a staff member offers him or her a wheelchair provided by the faith healing operation and seats the person near the front. When somebody comes in with their own wheelchair, they are given a position out of sight near the back. During the show, the healer, recognizing his own wheelchairs, asks the person to stand up and walk, which of course is something they can do. The amazed audience has assumed that the person in the wheelchair was somebody who actually needed it.
It is nearly impossible to avoid making assumptions which may occasionally be wrong. Our brain has to constantly make assumptions if we are going to respond to what is going on around us in a timely manner, and, of course, most of the assumptions are right. We should be aware, however, that we will occasionally be fooled, and we should be on the lookout for those occasions.