The earth rushing up to greet you at what seems like a million miles an hour?
The thought that your parachute could fail?
Having a heart attack mid-jump and failing to open your chute?
Somehow skewering yourself on a radio antenna?
Okay maybe that last one is a stretch. I've never wanted to go skydiving, mainly because I'm afraid of heights and I get that weird "Oh god I'm going to fall I'm definitely going to fall it's only a matter of time" feeling when I'm on a ladder at my house, let alone a thousand feet in the air staring out an open door on a plane. I went bungee jumping once when my pal Tim worked at a theme park in Florida. Despite the fact that I watched roughly a dozen people do it before me, when I got all roped up and to the edge of the platform it took me probably a solid 15 minutes to force myself to walk off the edge (I'm glad I did it, but it wasn't thrilling enough to make me want to do it again).
But never in my most acrophobic nightmares have I considered the possibility of this happening: