J.R.
My dad had an episode of transient global amnesia at age 53. Luckily, his employees knew his behavior was abnormal-he was repeating himself a lot in a meeting-and they called our family. We immediately took him to the doctor. The tests (EKG, CAT, MRI) showed no reason for the episode, and we have never found out the cause. He was completely normal within days; he just couldn't remember the hours involved.
My dad's research on the topic showed that evidently, people who experience this have high IQs. :)