h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Actually, I'm not sure if it was a janitor who did this, but at the University where I work, no one is allowed to mix their own cleaning chemicals (not even to mix down concentrated cleaning solutions with water). There is a department which does the mixing and supplies the mixed chemicals to other departments.

Why is this? Well, what they told us was that at some point a few years ago, some employee went to clean up something, and decided that since bleach and ammonia were such good cleaners that mixing the two together would be an even better cleaner.

Needless to say, a big cloud of chlorine gas came up out of the bucket. He couldn't breathe, so he came up with the brilliant idea of turning the fans on, which blew the stuff throughout the building.

One evacution and several hospitalizations later, they came up with the "no mixing" policy...

When I was at primary school a staff member at our local swimming pool did a very similar thing to clean the toilets in the changing rooms. A cloud of chlorine gas blew over a swimming pool full of seven year old kids, hospitalising about fifty of us - it was hilarious...

That's... actually... quite funny. Sorry, I have a warped sense of humor.