Oh man, I've got way too many of these, but here are a few:
Best that didn't happen to me was during American Werewolf in 2013. There was an incredibly timid person in front of me, and as we got to the subway, they noticed the werewolf shadow creeping down the curved wall, and immediately clung to the wall opposite, where the subway business victim scared the daylights out of them, which naturally ping ponged them right into the werewolf scare diagonal from that. I have never seen a scare volley work that perfectly before or since!
Another great one was in 2014's Halloween. My friend's little sister was attending for the first time, and we were in the closet scene where you progress through the multiple versions of the closet, each time with Michael getting closer. The building tension was getting to be too much for her, and so when she finally saw the closet exit, she kind of bolted (as much as you can in a conga). Little did she know there was a Michael immediately upon exiting, and so he lurched forward and I swear it was like a cartoon: her upper torso froze immediately in place, but her legs continued running forward, which resulted in her briefly all but horizontal in the air, before she slammed down on the ground horizontally. She just kind of stared up at him, and the poor Michael didn't know what to do, so I scooped her up and moved her into the next room. (she was fine and laughed about it so hard after, but I can't help but think of Scooby Doo when I think of it!)
For me, that same year my first ever house was American Werewolf, and you never forget your first HHN scares (my brother and I left the house in each other's arms, out of breath, and a little hoarse), but the biggest scare of that event and to this date belongs to the ceiling dwelling zombie in Cabin in the Woods. I was so on edge in that house a pin drop could set me off, and so when the ceiling scare happened, I didn't immediately realize where it was coming from, I only heard the burst of sound. I have no explanation why, but my first instinct was to leap up a tiny bit, and literally mid-air I realized the zombie was above me, so without thinking I tried to redirect my mass downward, which resulted in me SLAMMING myself into the ground with about twice my own body weight, spring boarding into the next room! Fun times, but no sprained ankle at least!