In 1976, I was at the drive-in with a girlfriend and a buddy and his girlfriend, watching "Carrie". We had all had a few beers and were mellow. Right after the big terrible prom scene, the movie cuts to an epilogue shot of Amy Irving putting flowers on Carrie's grave. Suddenly Carrie's arm bursts out of the grave and grabs Amy. It's a real jump scare.
Right at that exact moment, someone walked into the speaker wire and the speaker lurched forward on the window with a BANG! We all screamed like girls and splashed beer all over the place.
Then we all laughed until we couldn't catch our breath.
Rosco