a very bright person can function far below his potential because his mind is elsewhere.
I like the one from W.E.B. Griffin, "room temperature IQ"

This reminds me of an incident that happened to me many years ago, when I still had my full mental capacity, when I attempted to join Mensa. The person in charge of the local branch asked me if I had ever taken an intelligence test. I assured him that I had, and had scored a 63. He then commented that Mensa was a high IQ society, and that 63 was well below their level. I retorted, "That's 63 Celsius, not Farenheit!"