We filmed a scene about the main characters hanging out in one's basement, and at the beginning of the scene Zac is trying to throw chips into Keith's mouth. On paper it wasn't really meant to be all that funny and I thought it'd go quick.
But, when we actually tried to film it, it was impossible not to laugh. When Zac would throw the chips, they would either go flying across the room, or they would smash against the wall right behind Keith's head. After about ten takes and littering the room with Doritos, we finally got a shot where someone wasn't laughing.
Also while filming the scene, our boom man accidentally dropped the boom pole on not one, but two of the actors' heads - but the audio all sounded great.