Poisons. Anything with a warning label on it. Not so bad about it anymore, I've had a lot of time to come to terms with it. It used to drive my dad crazy though, since he was a jack of all trades, and had me helping him quite often. I remember him handing me a jug of muriatic acid once, a pretty powerful cleaning agent. I dropped it as soon as I saw the skull and crossbones on it. Coulda been a big big mess. He was righteously pissed.
I know exactly what it stems from too. When I was a kid I was into chemistry, wanted to be a scientist. I was about 7 years old and was trying to recreate an impossible experiment that I saw in a Batman comic book. I had glass cigar tubes and everything I could find under the sink in the bathroom, cologne, soap, face powder, various cleaners, etc... I mixed potion after potion, trying to get that frog to turn into a vicious monster, but nothing was working. In the shaking of a "test tube" I accidentally got a little of the stuff in my mouth, it tasted like hell and it burned. Scared the crap out of me. I didn't know if I was going to die by poisoning, or turn into a vicious monster. Luckily, neither happened, but the experience left it's mark.
The frog didn't fair too well either...