Or only one performer do it. Some skill or trick or just something noticeable that you never saw repeated by another performer or in another scene. Of course nobody has seen every porn scene ever made, so corrections are welcomed too.
Here's mine.
I saw a Chessie Moore pretzel scene where Peter North buttfucked her (she's laying on a desk, he's standing) to completion, and came on her tits/face.
Every other pretzel scene I've seen, they just do it for a brief "Gee that's interesting" moment (no fucking at all) and/or quickly move on to other positions. Never seen it used as the scene climax.
Veronica Hart, was laying on her back with her legs up high, knees way up by her shoulders, geting fucked. Whenever the guy thrust deep for full penetration, she bent her knees so her legs were straight, essentially folded in half. What was unique was the knee flexing in tempo with the fucking. And it was shot from the side so it looked even hotter. Never seen it since, but afaik, I saw Veronica do it in other scenes.

Edited by - ninja1 on Jun 25 2010