Mine is the time I tried to convince an anti-abortion person on Facebook that when she claimed some ABSURD number of abortions were taking place that it was not possible. Whatever figure she gave, I took it and did some quick guesstimating math, showing my work, and it led to the conclusion that if her figure was right, around 50% of women on earth who were capable of pregnancy were not only getting pregnant every year, but also were having at least one abortion per year. She was completely undeterred by the fact that she probably was overestimating the number of abortions occurring by like 30 times or probably much more.

That's when I realized that facts are not important to certain people. 30 times overestimated or not, didn't matter. Even one was enough to give the exact same weight to the issue.