I have this condition. In both thumbs. Think I'm going for another cup of tea now.
The thumb when extended (as in a "thumbs-up") can extend backwards toward the nail and outwards, a recessive congenital condition known as "hitchhiker's thumb", whereas for other people it will extend straight out with little backward bending. Having either condition appears to have no effect on the thumb's function. While most people have either "hitchhiker's thumb" on both thumbs or neither, in some people, the condition only presents itself on one thumb.
Photo courtesy of Wikipedia.
17 February 2009
And speaking of my thumb...
My fun fact for the day from Wikipedia: