Many are familiar with the story from John 8, as follows (actually read this to the end, it has a point):
"1But Jesus went to the Mount of Olives.
2 Early in the morning He came again into the temple, and all the people were coming to Him; and He sat down and began to teach them.
3The scribes and the Pharisees brought a woman caught in adultery, and having set her in the center of the court,4they said to Him, "Teacher, this woman has been caught in adultery, in the very act.
5"Now in the Law, Moses commanded us to stone such women; what then do You say?"
6They were saying this, testing Him, so that they might have grounds for accusing Him. But Jesus stooped down and with His finger wrote on the ground.
7But when they persisted in asking Him, He straightened up, and said to them, "He who is without sin among you, let him be the first to throw a stone at her."
8Again He stooped down and wrote on the ground.
9When they heard it, they began to go out one by one, beginning with the older ones, and He was left alone, and the woman, where she was, in the center of the court."
John 8:1-9 (NASB)
There are many theories as to what Jesus was writing when he wrote on the ground, and I have a theory I would like to present. In Jeremiah 18:13 it says, "O LORD, the hope of Israel, all who forsake you will be put to shame. Those who turn away from you will be written in the dust because they have forsaken the LORD, the spring of living water." I believe the Pharisees were amoung those who have "foresaken the LORD," and thus Jesus was writing their names one by one in the dust.