France is bacon... Just kidding. Not exactly mishearing, but I misunderstood "the best defense is a good offense" as if you defend super well, you'd attack your opponent with your defense. It didn't make much sense until I realized you're supposed to attack in order to defend...
Can't think of a specific line, but I'm sure something from Grease. I was obsessed with that movie as a kid, and my many questions about some of the plots and lines led to my parents giving me The Talk. So I imagine I misheard a bunch of lines about periods and such.