I attended a talk by a gifted specialist earlier in the year who covered this exact thing. The strategies he suggested have pretty much been covered by other posters (pre-check etc), but I thought I'd just add that he felt it was very common in gifted kids and that it is more of an issue the more global/holistic a child's thinking style is. They just don't pick up the detail in the question (and others have already mentioned, he felt it was much more likely to happen when the work was too easy).
Don't know that that is helpful - just to let you know it's not uncommon
