"Sometimes I have difficulty sleeping at night," reflected former German Prime Minister Fritz Haber, "but I find comfort in knowing that I did what was best for my people, even if I had to hurt some of them in order to do so."
^^ was my "hook" for one of the AAMC FL essays. Invented on the spot, particularly if you google 'Fritz Haber.' I've invented counterexamples before and realistically, I say that there's only one rule: make it believable.
If you go to the trouble of thinking up the ideal hypothetical example for your essay, and then flesh it out by sprinkling some names, dates, or locations, then you show that you have the same level of logical reasoning as any other candidate. So long as the examiner cannot positively identify it as being false, you're home free, so make sure that it really is the best possible example for your argument and go to town.