Not quite the same but having listened to the 1712 Overture several times before hearing the 1812, I didn't get some of the jokes until I actually heard the 1812, and THAT's when I laughed.

But on-topic me and the wife have been watching an animation film where the cinema was mostly kids and we got, and laughed out loud at, some of the "jokes for the grown-ups" that none of the kids did, who were all staring at us for laughing at something that was totally not funny for them.

Not a line per se, but a scene. Watching The Haunting of Emily Rose in theaters and it's supposed to be a serious scene and maybe even shocking or scary. The possessed main character jumps out of a barn loft or house window... I lost my shit, laughed so loud. The way the girl jumped out was just too funny looking.

I DID NAWT HIT HAR. I DID NAWT. OH HAI MARK!

I got lots of weird looks for laughing during multiple death scenes in the John wick movies, not sure if that counts...

I used to laugh during certain scenes in Commando. He'll do ridiculous things like rip the seat out of a truck like it's nothing.

The times that this has happened to me weren't serious moments that tickled me wrong, they were sophisticated references that only I got (or others were just quieter). I couldn't give you a specific example but it's happened at least twice.