"Now Abimelech had not come near her (Sarah); and he said, 'Lord, wilt Thou slay a nation, even though blameless?
"'Did he not himself say to me, 'She is my sister'? And she herself said, 'He is my brother.' In the integrity of my heart and the innocence of my hands I have done this.'" Genesis 20:3-5
What Abimelech said was true. This was not one of Abraham's shining moments. Sarah was so beautiful he knew every nation they went into the king would want her for himself (she must have been so breath-taking -- and she was OLD) so he told her to tell everyone they were brother and sister rather than man and wife. He had his own person in mind here. He wanted to protect himself (having no thought for Sarah).
Abimelech appealed to God. Now, what I think is cool is God's answer. Here's what God said to him in the next few verses:
"'Yes, I know that in the integrity off your heart you have done this, and I also kept you from sinning against Me; therefore I did not let you touch her. Now therefore, restore the man's wife, for he is a prophet, and he will pray for you, and you will live. But if you do not restore her, know that you shall surely die, you and all who are yours.'"
Notice what God didn't say -- He didn't agree with Abimelech about Abraham. He didn't even mention Abraham except to say he was a great prophet. He kept the talk centered on Abimelech. God kept the conversation on Abimelech's faith and response.
God doesn't say negative things about His children to His other children (even if they are messing badly). God didn't even acknowledge Abimelech's arguments about Abraham and Sarah. He could have.
Instead, God called Abraham a great prophet.
When you hear complaints about another person, do you listen and agree or say something about that person in sympathy with the other person? God doesn't do this.
(This doesn't apply to every single situation but to most it does)