I think that one of the worst situations is when enrichment occurs as extra homework, but the original boring drills still need to be done too. Luckily we haven't had to deal with that much yet-- I tend to just tell DS to ignore the busy work, but that approach might cause minor strife with rigid schoolteachers and admins. Still, there's no way I'm going to let the school take more time out of my son's day because he's gifted.
One thing I really dislike about our current school situation regarding math is the period at the start of every year where the almost exclusive focus is on drilling on basic facts, to get ready for testing and combat the district's historically low math scores. I believe this is an approach recommended by the district's math consultant, of whom I have a low opinion in general. This winds up in reducing even further the time spent each year on instruction on new skills and concepts, and thus negatively impacts all the children who are strong in math.
