Nothing inconsistent with the rest of His teaching.

    And as they continued to ask him, he stood up and said to them, “Let him who is without sin among you be the first to throw a stone at her.” And once more he bent down and wrote on the ground. But when they heard it, they went away one by one, beginning with the older ones, and Jesus was left alone with the woman standing before him. Jesus stood up and said to her, “Woman, where are they? Has no one condemned you?” She said, “No one, Lord.” And Jesus said, “Neither do I condemn you; go, and from now on sin no more.”  —John 8:7-11 ESV

J.C Ryle:

Our Lord knew the hearts of the malicious questioners before Him, and dealt with them with perfect wisdom, as He had done in the case of the “tribute-money.” (Matt. 22:17.) He refused to be a “judge” and lawgiver among them, and specially in a case which their own law had already decided. He gave them at first no answer at all.

But “when they continued asking,” our Lord silenced them with a withering and heart-searching reply. “He that is without sin among you,” he said, “let him first cast a stone at her.” He did not say that the woman had not sinned, or that her sin was a trifling and excusable one. But He reminded her accusers that they at any rate were not the people to bring a charge against her. Their own motives and lives were far from pure. They themselves did not come into the case with clean hands. What they really desired was not to vindicate the purity of God’s law, and punish a sinner, but to wreak their malice on Himself.

Last of all, when those who had brought the unhappy woman to our Lord had gone out from His presence, “convicted by their own conscience,” He dismissed the guilty sinner with the solemn words, “Neither do I condemn you–go and sin no more.” That she did not deserve punishment He did not say. But He had not come to be a judge. Moreover, in the absence of all witnesses or accusers, there was no case before Him. Let her then depart as one whose guilt was “not proven,” even though she was really guilty, and let her “sin no more.”

To say in the face of these simple facts that our Lord made light of the sin of adultery is not fair. There is nothing in the passage before us to prove it. Of all whose words are recorded in the Bible there is none who has spoken so strongly about the breach of the seventh commandment as our divine Master. It is He who has taught that it may be broken by a look or a thought, as well as by an open act. (Matt. 5:28.) It is He who has spoken more strongly than any about the sanctity of the marriage relation. (Matt. 19:5.) In all that is recorded here, we see nothing inconsistent with the rest of His teaching. He simply refused to usurp the office of the judge and to pronounce condemnation on a guilty woman, for the gratification of His deadly enemies.