Here is one:

http://www.hoagiesgifted.org/tests_tell_us.htm

..."Group tests are generally normed on populations of all children, with relatively few gifted children among the mix. When taking group intelligence tests, gifted kids often "over-think" the questions, and perhaps make wrong selections. And since there's no individual tester to clarify unusual answers, the gifted kids often score lower on group intelligence tests."