I remember in Revenge of the Sith, when the actor playing Vader yells "nooooooooo", at what is supposed to be the emotional climax of the trilogy, the theater roared in laughter.
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.