When I was an undergrad, my advisor was once cleaning out a lab. Like all labs, there were, of course, some unlabelled bottles full of clear liquids.
(It is an invariant rule, in labs of all persuasions, that unlabelled anything should go for hazardous disposal by experts. Because really, you don't know what it is. Nobody actually does this because usually it's TBE or salt water or something lame. My father, who is a safety consultant, then makes a fortune cleaning up after their disasters.)
She opened one such one-liter bottle of a clear liquid, sniffed it delicately, and declared, "I know what this is!", thinking it was hydrochloric acid. (HCl has a very distinctive smell.) So she turned the cold water on full blast and poured it slowly down the sink....
... and then they had to evacuate the building. Because it was concentrated ammonium chloride, which smells just like HCl, but releases ammonia when it hits water.
When I was in grad school, I was once cleaning out the hood. I opened a Falcon tube full of a clear liquid, thought "I know what this is!" and dumped it down the sink. It was also ammonium chloride.
Last month, my dear spouse was washing the dishes and cleaning the kitchen as he does every night. He saw a big pot full of a yellowish liquid and said, "I know what this is!", thinking it was water from boiling something. So he dumped it down the sink....
... it was 83 ounces of oils for soapmaking.
(I will get back to you with the eventual plumbing bill.)