In my opinion the events where you could get seriously hurt would have to rank as the hardest. That being said, here is my list:
1) Pole Vault
2) Steeple/Hurdles- Jumping and falling, enough said
3) Hammer- If the wire snaps when you are turning, you are probably on your way to the hospital. Plus, most people who have never done it (and try to seriously throw it) almost always fall down.
4) High Jump- Take off to soon and you are probably off to the hospital
5) Triple Jump- If your knee buckles, you are probably on your way to the hospital.