We had this same problem in our house. One of the reasons we saw it was more evident was in the lack of challenging work. We have since been accelerated a grade and this is much less of a problem. Sometimes I think the work is so easy that they often rush through it skipping over things.
We still have it happen on occasion (but again, it's usually on the things that were "review" and they were careless errors.)
I think sometimes their brains just move so fast they often overlook things. DD will do the same thing and we too, "Is that right? What's wrong with it? YOu find it." She would often "check" her work and couldn't find the mistake because she would be so sure what she did was right and would check her answer but not rework the problem without looking at what she orig. put. Again, it's always on things that are "too easy" that we make the most mistakes (careless errors, not lack of knowing how to do it. that's the difference!)
