I suspect the answer will vary based on the children's ages. My recollection is that I made very different mistakes in 10th grade than I did in 2nd or 6th.

Misunderstanding the question, for instance, may show up more at later ages as the questions get less cut-and-dried. Forgetting the rote-memorized multiplication table may show up most at young ages. Misremembering and misapplying the rules for symbol-pushing may show up more at middle ages.