Mine is from when I was 14:

I mixed calcium carbide with water inside a glass bottle. Then I closed its lid. Then I waited until I got really concentrated acetylene. What I got was a scar on my right arm, a smaller one just above my upper lip (nowadays hidden by the beard), and a big scratch on my prescription glasses — without them I'd be probably blind from my left eye.

From that I've learned some valuable things:

1. I'm a muppet.
2. I'm a bloody muppet.
3. My mum was also a muppet, for letting me fuck with calcium carbide, sodium nitrate, concentrated sulphuric acid, sodium hydroxide, concentrated ammonia, gunpowder etc., since my teen years. (Guess where I got the calcium carbide from? Her brother's garage!)
4. My dog (rest in peace, Lana; you were the greatest girl) was probably traumatised with loud noises because of me. Now thinking, Lana was also with me the time I melted lead and poured sulphur on it, and instead of getting galena I got a whiff of Hell on my face.
5. You can tell people a different story every time they ask you about the scar, and they'll buy it. The one I just told was the true one, though.
6. Glass containers are fragile from the inside.

Anyway, that's my "nitric acid acts upon trousers" moment.