What was the most tragic incident in Horse Racing?

It has to be any one in which a human life was lost. Many jockeys have been killed while riding a race.

With regards to horses, any loss is also tragic. But some of the most famous champion horse who died from injuries suffered on the track include Ruffian, Go for Wand, Eight Belles, and Barbaro, who died 8 months after his injury.

In the related links box is a link to a story on the horse racing industry's reaction to the death of Eight Belles in the 2008 Kentucky Derby, as well as a discussion of some of the incidents referenced above.