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.)

(In the 154th issue of the BE newsletter, another solution, noting that "Merab" appears in two Hebrew mss. and some LXX mss. in place of "Michal" is offered. Dennis McKinsey says of this solution:

It doesn't take a rocket scientist to realize that the more opportunistic translators opted for the word "Merab" in order to escape this contradiction, even though they were fully aware of the fact that it is found in only 2 Hebrew and some Septuagint manuscripts, while all the other Hebrew and Septuagint manuscripts have "Michal." That's politics, pure and simple, my friend. Now you can see why I often call the Bible and its various versions a political book.

Of course there is not the slightest evidence that politics, rather than valid textual-critical procedures, is behind the use of "Merab". McKinsey seems to think that textual choices are governed by mere majority of readings. Such assumptions are of course exemplary of his ignorance of textual-critical procedures.)