Most people have encountered someone they think is overconfident. In many cases, individuals may recall cases where they were overconfident themselves. As perhaps an overly skeptical individual, I can only recall two cases where I considered myself "extremely confident" and was wrong. These were humbling experiences, and I tend to avoid being overconfident, which may explain my lack of experiences with overconfidence steering me wrong. Nonetheless, psychology has revealed what's called the "overconfidence effect" that sheds light on the fact that more often than one would expect, people who are "extremely confident" are wrong.
More specifically, individuals were studied in a variety of ways. There were questions given to people where the average frequency of error had already been determined. After asked their confidence level in the answer given, the numbers are compared. Those who considered themselves 100% confident were wrong 20% of the time. In another scenario, psychology students were asked to give confidence intervals with respect to how they would do on a multiple choice test. Especially given the increased likelihood of awareness regarding the effect itself, the students were still overconfident given the results. When given more information, confidence levels rose while there was no significant increase on success rates.
The studies of the overconfidence effect suggest that even though information may not be particularly useful to the subject at hand, it provides individuals more confidence in their views. The widespread skepticism of popular scientific views, for instance, might be explained in terms of massive exposure to false information. Because individuals think they have become informed, they are more confident in their beliefs. Furthermore, it seems to be a consistently observable phenomenon (at least in the majority of people). In that sense, individuals may be confident to improve success rates, perhaps, even though the confidence levels are still reflecting unfounded certainty.
In daily life, individuals may want to reflect upon their certainty in terms of "is this justified" and "even if it isn't, should I believe it?" For instance, you might not be justified in believing that as the underdog, you will win. However, you need to recognize that you are in control of your own effort and positive attitudes will increase the chances of team success. In that sense, you have to accept that you "will win" even if it is overconfident. Alternatively, arguing over who left the door open may be pointless as it's more than possible you are remembering incorrectly. For instance, I know someone who occasionally uses the phrase "100% sure," and I've never heard them use it and not turn out to be wrong. So overcompensating for a lack of evidence to justify one's position may be another explanation for the prevalence of the effect. Either way, it provides useful information for all critical thinkers by reminding them to carefully reflect on their own beliefs and avoid being excessively confident when there is no justification for it. Or if you're like Socrates, avoid overconfidence entirely.