Sometimes it's all about working fast pace action and humor together, or even just plain randomness. I could give example with my story "Can a Lion drive a tank?", but it is a little foul in the language department. The best way for me to right a comedic scene is first type out a quick over lay that gets you to giggle then expand a bit at a time. Have a good grasp of the idea as you do so you don't lose the feeling of the scene and make sure that it makes you laugh firsthand. The hard part of a really good comedic scene is that what works for one person doesn't work for everyone. well I'll leave the address to my story to see what I mean. maybe it will help you out some.