20 Things You Didn't Know About Serenity
6. “Summer!”
While filming, whenever actors would flub a line, they would yell “Summer!” in anger.
This gag originated while filming the TV series after Summer Glau forgot her lines during a long and difficult scene.
You will notice that Morena Baccarin, Nathan Fillion and Sean Maher all doing this during the DVD blooper reel.