I immediately see red flags when someone presents themself as absolutely certain of their pronouncements and more so when they repeatedly show that there is little or no correlation between their confidence level in making a particular statement and the amount of evidence they have or the amount of work they put into determining their conclusions. The truly unfortunate part, in my experience, is that managers typically see this confidence as a surface level indicator that these people know what they're talking about and give these unreasonable people the authority act on their over-confidence.