Probably because I accidentally made chlorine gas.

You tried to electrolyse table salt, right?

I did the same as a kid. The chlorine reacted with the transparent plastic from my electrolysis apparatus, and made one side opaque and ugly, I hated it. (The “electrolysis apparatus” was a transformer, two pencil leads, two plastic bottles, a bunch of aquarium tubes, and glue. Lots and lots of glue. Even then it leaked a lot, it was hilariously unsafe.)

Then I learnt I should care about the electrolyte, and started using lye. (Nowadays I wonder what my parents had in their heads to let me play around this sort of stuff. It’s surprising I’m still alive.)