CAT Scans. They're a pain in the ass.

Still a nightmare inducing fear.

I remember that when I was very very young, I used to fear going to the toilet. Yes, it sounds silly, but I did had problems. I went to the toilet like once a week, and when I did, it hurt like HELL. Literally, a pain in the ass. But later, I started eating more fiber, fruits and random stuff, and I could go in peace to the toilet.
But still, I remember fearing that moment.

I hurt myself so bad in the same way that now I can't stop eating dry cereal.

I used to have the same problem: juvenile constipation. Sounds silly, but I had that fear. Much pain...

It's the worst thing that can eventually happen to you.
That feeling of "HRRGGGGHHH ARRRRGGGHHHHH HAAAAAAA!!!" and having no results, and when you finally can, it hurts so bad.
Almost like having a baby.

I fear girls, not because I'm shy/gay, but because they have cooties!

No, but on a more serious note, I fear creepers in mine-craft. They are so quite, and strike so fast.

JK, my greatest fear is; What if our entire life is a lie? We are part of a social experiment, or in the matrix.

Wouldn't make it any less real to us.
Reality is perception.

I've been thinking about that a lot. Even if we are such an experiment it, as Lizard said, wouldn't change what it means to us.
Also, you can't prove we aren't oranges from Alpha Centauri imagining all this.

We should be proud that someone or something cares enough to sustain us as we are, not be afraid of our own unproven existence.

I fear diseases like cancer and stuff.
Anything painful with a mortality rate of more then 50%.

I used to be quite phobic on the possibility of dying without having any offspring.