Mine is phenolphthalein, a chemical used as a pH indicator in my high school chem class.
Anyone who’s worked around phenolphthalein knows its bitter chemical odor. They also know it as being rather messy and, for my clumsy hands, hard to work with without spilling. But hell, it does what it’s supposed to do, and when you’re titrating a solution, anything to make a touchy process more exact is welcome.
Getting on with my story, I was working in the lab in full regalia, a setup that didn’t include gloves. I’m not sure how it entered my mouth, as a good scrub was SOP after any lab, but it might have been absorbed through the skin of my ungloved hands. I do know I didn’t get my hands near my mouth: Nobody could have paid me enough to even risk getting any of that vile stuff on my tongue.
Of course, I didn’t notice anything odd until I got home. Then I began to taste in my mouth the bitter chemical smell associated with my Least Favored Indicator. Not lightly, mind you, but very strong. Overpowering. Enough to make me need to drink large amounts of water to attempt to remove the taste from my olfactory system.
The taste is indescribable, but it can be approximated: Lick copper (but don’t eat it, as it’s a neurotoxin). Copper is bitter the same way as phenolphthalein is bitter, but it lacks the chemical tones that make the indicator’s taste that much worse. I know people who ate pennies when they were children, but nobody would deliberately drink phenolphthalein.
Aside from the taste, my extremities began to get cold. Icy cold, even as I lay in bed, wrapped in my covers. This also points to The Big P: As the MSDS linked to above indicates, phenolphthalein lowers your blood pressure.
Of course, the MSDS recommends inducing vomiting. I didn’t. It recommends seeking medical attention. I didn’t. I survived, but it isn’t something I’d volunteer to undergo again.
As a final note, phenolphthalein is carcinogenic. Did my teacher mention this? Nooo. So when I turn into a giant walking tumor with cold fingers and toes and a truly awful taste in my mouth, I’ll know who to terrorise. 
