Courtesy of Murderati.com:
How many screenwriters does it take to change a light bulb?
1st draft. Hero changes light bulb.
2nd draft. Villain changes light bulb.
3rd draft. Hero stops villain from changing light bulb. Villain falls to death.
4th draft. Lose the light bulb.
5th draft. Light bulb back in. Fluorescent instead of tungsten.
6th draft. Villain breaks bulb, uses it to kill hero's mentor.
7th draft. Fluorescent not working. Back to tungsten.
8th draft. Hero forces villain to eat light bulb.
9th draft. Hero laments loss of light bulb. Doesn't change it.
10th draft. Hero changes light bulb.