I thought of antifreeze because after an episode of "True Crime," where a wife killed her husband with antifreeze, I was thinking, "It must taste terrible; how could he not have known?"
Then I researched it & found that antifreeze has an interesting history - it's been frequently used as a murder weapon by wives who want an early payout on their husband's life insurance. It previously had a pleasant, sweet taste, so it was undetectable in tea, jello, anything that's expected to taste sweet. It was also difficult to detect unless the pathologist looked for it; the death looks like a heart attack unless the kidneys are cut open to reveal some type of crystals caused by ingestion.
They had to add a chemical to antifreeze to make it taste bitter so it couldn't be used to kill.