A traffic jam.
or
Three skunks fighting over a pickle
First dog: My master calls me Furball. How about you?
Second Dog: My master calls me Sitboy
or
Three:
One to write the light bulb removal program,
one to write the light bulb insertion program, and
one to act as a light bulb administrator to make sure nobody else tries to change the light bulb at the same time.
or
An envelope.
or
I was amazed, I never knew they worked.
or
They screw millions of bulbs every day, but when it comes to your bulbs, there's no record.
or