I recently questioned my understanding of multiple comparison when I saw this xkcd comic: https://xkcd.com/882/.
The comic is illustrating multiple comparison by showing a study that concludes that green jelly beans cause acne. It is illustrating that, by testing 20 different hypotheses with the same $\alpha$ level (0.05), it is highly likely that a false positive conclusion will be drawn.
What I don't understand, is why 20 different hypotheses are the cause of this. Since these hypotheses about the colors of jelly beans are independent, and the testing of each of those hypotheses do not influence each other, it doesn't seem to me how this could be an issue?
More precisely, I understand why it is an issue if the same hypothesis is tested 20 times, e.g. the hypothesis that "green jelly beans cause acne". But, in this comic it is only evaluated once. The hypothesis "jelly beans cause acne" is tested 20 times, but we don't draw a conclusion on that, we only draw a conclusion on green jelly beans.
It doesn't make sense to me why evaluating a hypothesis once is considered multiple comparison?