Overconfidence can affect us in many ways, but in my opinion, it's more aligned to the fact that it mainly comes hand in hand with underestimating others or certain situations.
I've dealt with overconfidence a lot in my life and they ways it managed to backfire to me are countless; My studies were very affected by my overconfidence, I used to spend little to no time studying because my Modus Operandi of thinking back then was "Damn i'm so cool and smart, I don't have to study that much at all." And then I'd end up getting lower than average grades and looking like a fool to my friends, teachers and my own parents as well, but most importantly myself.
Another way overconfidence can affect you is in terms of punctuality, and this still affects me on a daily basis (Something I gotta fix...) So in here you belittle whatever variables could affect you or your medium of transport in any sort of way. Thinking in a way of "Oh, I can probably get there on time" and then not being able to is completely disrespectful for your own persona. You're left with the reputation of not being able to attend commitments or date you set out for yourself. And again, you're being overconfident by underestimating outside variables that can affect you; There could be a car crash in front of your house, the bus could decide to come an hour later, your car might break down, etc.
Those examples ARE the ones which have affected me the most in terms of overconfidence. I would like to define the word in my own terms as "To think you can handle everything the world has to throw at you." which is obviously a lie; There are absolutely a very high number of things we probably can't handle out there in the real world and we should aknowledge this.
As a conclusion I think overconfidence leads to Recklessness and as a direct effect of this cause, we're left with a complete situation we're not ready to deal with. So the moral of the story is that it's okay if you're confident in yourself, but its always better to be cautious and prepared than to be bold and blunt. It's better to prevent certain situations than later be sorry for them even happening at all.