When Harry, Ron & Hermione were discussing Harry's kiss with Cho, the 3 began to crack up at the end of the scene.
That was real laughter from the 3 actors and the director decided to keep on rolling because he felt it was a Happy Accident.
***
If you guys don't know what a 'happy accident' is, it's a stage word for 'you messed up but you made everyone laugh so it's okay'.