It is a very common happening amoung very bright people. It tends to happen most when they are trying their best to perform well.
I had a friend that had an I.Q. of about 180. He was unable to pass an oral exam on a subject he likely knew better than all his instructors combined. He just mangled everything. I finally talked him into going with out sleep for 48 hours before he took the test again. He was totally exhaused and aced the test with a perfect score. We just had to dumb him down to the level of the people who created the test, and exhaust him so his brain slowed way down. It worked. He performed perfectly.