The worst coffee I have ever been served was actually in the cafe at the Swindon Steam museum about 5 years ago. I suspect they had used some cleaning solution in the coffee machine and hadn't flushed it out properly. It tasted really horrible and I shudder when I think about it to this day!
My old boss got similar in our office. My colleague had decided the office kettle was getting furred up, so filled it with descaler. Then he went out. Enter the boss, who took his coffee black, and was delighted to the kettle was full...
The office closed some years ago. Those who worked there still talk about this. Not sure what happened to the boss.
Phosphoric acid is the usual material used to remove limescale from kettles, coffee machines, water boilers and the like. It is only moderately toxic and unlikely to do any harm if ingested, unless in improbable amounts.
It is one of the ingredients in cola drinks.