In 1848, Gage was a railroad worker who had the sort of job that sounds like it was designed for the Darwin awards: he was paid to drill holes in large rocks, fill them with gunpowder and pack it down with a large iron rod.
Not surprisingly, the gunpowder eventually ignited, sending the tamping iron through Gage’s skull.
Remarkably, Gage survived, but not without significant damage to his frontal lobes.
Gage seemed to show some changes in character (although the exactly details are still somewhat controversial), and this was one of the first clues that specific areas of the brain may be involved in specific mental functions.
More recently, scientific studies have been completed to work out the path of the iron through his skull, to understand exactly how the brain was affected.
Neurophilosopher has video of the computer reconstructions created by these studies, and discusses some of the historical details of the incident.