Overconfidence can negatively impact the way you handle your expectations, actual results and how others can perceive your actions. Being overconfident about something does not necessarily mean being capable of handling whatever the task may be.
Overconfidence may lead the person to lose their objectivity and make decisions without thoroughly thinking things through. They just have faith that their decisions are objective and on point when it is not really true.
It can lead to creating unrealistic expectations of results that have yet to be observed. This may lead the overconfident person unable to handle failure especially when they are sure about their decisions.
Overconfidence may also negatively impact the way a person relates to others as coming off being boastful when the person does not intend to.