The Vampire Diaries -- Katherine's Deal With John
On the surface, what Katherine told Damon seems pretty straight-forward. She cut a deal with John Gilbert to get out of the tomb that entailed one of the Salvatore brothers dying, and when given a choice she chose for Damon to die and Stefan to live. However, when you start thinking about it, there's a lot of discrepancies in what she said and you start to wonder just what the truth was and what was a lie.
One thing that is obviously a lie, which was the most painful thing she said to Damon, was that she had a choice and she picked for Stefan to live and Damon to die. Before John even came to her in the tomb he gave the dagger to Damon to kill Elijah with. So John didn't give her a choice; he made the choice himself of which Salvatore brother would die.
That brings us to the other biggie: did she really know Damon would die if he used that dagger to kill Elijah. I think it could go either way on that one. Damon was the one who put her in the tomb and she may have been testing him when she lied to him that if he killed Elijah she'd be trapped in the tomb, forever. When Damon didn't care she'd be trapped forever, she might have felt then why should she care if he dies by using the dagger. Fair is fair.
On the other hand, when Katherine was in cahoots with John and Isobel before and John tried to kill Damon, Katherine went after him with a butcher knife and tried to kill him. She also brought Damon the werewolf cure to save him and gave up her chance to be free of Klaus to save Damon's life. So it doesn't really sound like she'd be willing to let Damon die just to free herself from the tomb.
There's a possibility John could have lied to Katherine and she didn't know the dagger would kill Damon, explaining why she was waiting for Damon in his shower when she should have thought he was dead because she was able to get free of the tomb.
Another possibility was there was a loophole around the killer dagger. Maybe that's why John showed up uninvited to the dinner party Damon threw where he planned to use the dagger. Maybe he knew of something that would revive Damon after he used it. If you had set someone up to be murdered, would you really go there to watch it happened, when someone there could kill you in response?
The final oddity is why Katherine didn't want John or Isobel to know she was still in town once she got out of the tomb, when she claimed staying in town to help deal with Klaus was part of the deal she struck with them. Ultimately, she join forces with Isobel against John, but why didn't she want John and Isobel to know she was still in town?
The truth will never probably be revealed because this is but a minor blip in the story. There's still so much of Damon and Katherine's past that is cloaked in mystery that will probably never be revealed since Julie Plec thinks the only important history was Stefan and Katherine.
Still, it makes you wonder what is the truth and what's a lie.