The warm up of a lesson often receives less attention than it should. Teachers spend a lot of time preparing explanations and worksheets to introduce and practice the target language, for example. They then enter the classroom unprepared for the first five or ten minutes. "Let's do something fun" usually constitutes all the planning that goes into this stage of the lesson. Planning then gets done on the way to the classroom, with the teacher pulling a game out of his bag of tricks.
Every teacher with more than one month's experience is guilty, including myself. But a well-planned, effective warm up offers more towards the lesson than just a bit of fun.