To BrockawayBaby (and those who commented with regards to Baby's first comment);
Passer-by is a hyphenated word.
Playoff (like blastoff) has been reduced to a single word, without hyphen.
The plural of passer-by is passers-by, just as simply as it was described above (although I do admit it even sounds strange at first). I mean, we can all see it. Mothers-in-law; anti-inflammatories, etc.
However, for words like playoff or blastoff, they are not hyphenated words, and as such, would only be pluralized at the ending. After all, in any other such case with compound words, when would you randomly insert an S into it? Breakdown would be breakdowns, not breaksdown; countrysides, not countriesside.
Anyway, correct me if I'm wrong and I know I'm 4 years late to this party...