In 2 Samuel 3:7-11, Ish-bosheth accuses Abner of taking one of Saul's concubines: Rizpah, daughter of Aiah. It seems that were this accusation true, it would be an affront to Ish-bosheth's throne. In the text, Abner bristles at the claim, emphasizing his loyalty to the house of Saul; yet, he doesn't outright deny the accusation. Did Abner go into Saul's concubine?
The three commentaries I've read are split on this, but none of them argue their side; they simply assume that he did or did not. My leaning is to say that he did not, since his response to swear his loyalty to David's house makes little sense if Abner himself has his sights on the throne. But it surprised me that others disagreed and now I'm not so confident I'm reading the story correctly.