Dude, I'm not sure which case you are referring to, but if it's the first case, I want to offer a note of caution.
It's not really over confidence as much as it is impulsivity born from an avoidance of some feeling which I'm not sure has a label. Whether it's understimulation they are avoiding, boredom, or the monotony of focusing on something simple when their mind is craving something else, I don't know. But I do know where it can lead.
When you don't follow directions or you don't read the problem fully, you get things marked wrong. Even if you read it again and again, you don't understand why you got it wrong unless you see where your mistake is. Then you either think the question maker was at fault, you think "well, I would have gotten it right so I'm still perfect", or you think that you are not competent at doing this work. My dd goes through all of these thoughts, but tends toward the third. When you have lost confidence, it can be seriously limiting. The areas where your talent is the strongest are the areas you avoid and you become a serious underachiever. The need to read critically increases as you move toward middle and high school, and bad habits from early elementary can really haunt you.
Your results may vary. In my personal experience, the results were that getting marked off usually forced me to figure out what I'd done wrong. If I couldn't figure out what I'd done wrong, I'd ask. Occasionally it wasn't my problem, but one where the teacher may have made a mistake, or where the question was terrible, directions poorly written, etc. Most of the time, the mistake was mine, and I owned it.
Over time, I gradually learned to read directions and/or check my work. That serves me well today, where I'm in a position where a simple error, a failure to follow instructions, or a failure to check my work can cost my organization millions of dollars. Over the course of my career, I've encountered numerous professionals who are so frozen by the fear of making mistakes that they're practically useless to their employers.
Mistakes are the best teacher on earth. Very few 8yos face the same kinds of stakes I do, and a little bit of failure can do them a lot of good.
