@BrockawayBaby

"The justifications of "passersby" as the proper plural is silly. I know the grammar tyrants like to think it's correct, but think about the following: the word "playoff" is made up, like "passerby," of a noun (passer) and an adjective (by). Under the grammar tyrant rule, the plural of "playoff" should actually be "playsoff.""

This is not a noun + adjective. It's a verb + adverb. The same goes for those other words in your list. 'Passersby' is correct, as is 'playoffs'.