Polychlorinated Biphenyls (PCBs) are neurotoxins and potent carcinogens that were once used extensively in cooling oils for electrical transformers back when we were fearless (and stupid).
Transformers can overheat and catch fire. Breathing the smoke from a burning transformer containing PCBs can put firemen at more risk than they signed up for.
At the train station in Hoboken, New Jersey, I spied a large transformer with a sticker on it that said:
What did this sticker mean?
Did a protestor place the sticker there thinking he was working in the best interests of workers, firemen, and the general public?
Or did it mean that the transformer had been tested and was free of PCBs?
A falsely reassure a maintenance person or a fireman might put himself at great risk. Or he might take it as a warning and fail to put out a fire putting others at risk.
These two words (well, one word and an acronym) take my prize for the shortest ambiguous sentence.
Don’t believe what you can misread.
Ambiguity is the soul of the nitwit.
 Replacing more than one of the hydrogen atoms in a biphenol with chlorine atoms produces a Polychlorinated Biphenol; but you knew that already just from the name.