2 Sam. 6:23 Therefore Michal the daughter of Saul had no child unto the day of her death.
2 Samuel 21:8 But the king took the two sons of Rizpah the daughter of Aiah, whom she bare unto Saul, Armoni and Mephibosheth; and the five sons of Michal the daughter of Saul, whom she brought up for Adriel the son of Barzillai the Meholathite:
This one's pretty simple: The second verse should have Merab instead of Michal. This change is attested to by two Hebrew manuscripts, some Septuagint manuscripts, and the Syriac version. We are also clued in by 1 Samuel 18:19, where it is noted that Merab was married to Adriel. (For general background, see foundation essay on copyist errors.)