I've just touched on middle age, and have been lucky not to have witnessed anything too horrible, or too often...but have three experiences in particular that shook me somewhat:
1) In April 2000, I saw the aftermath of a murder scene, where there was blood on the ground and cops everywhere marking out the scene (they caught the perp like the day after, so its not a cold case, at least)
2) Just over a year ago, I saw a motorcycle crash where there was the prostrate body of the cyclist and a trail of blood;
3) In 1998, I was walking down a main street about to cross a side street, somebody was walking in front of me; as he crossed the side street, the driver did a rolling stop through the stop sign w/o paying attention and smacked into the guy - I yelled STOP and banged his window as the driver was advancing and the victim got partially sucked under the car - he was rushed to hospital for trauma and likely a broken or very bruised leg...
Other than that, I can't say that I've had as jarring experiences as others - one of my acquaintances said he witnessed a guy do a suicide jump from a building, another said he watched his friend get stabbed in a nightclub and die beside him...yikes.
Wonder what you folks out there might have been witness to?