Mark 12:27 He is not the God of the dead, but the God of the living: you therefore do greatly err.
No, he is not a God of dead one but living ones: much have you been lead astray by yourself.
Christ actually sets up the wordplay here back in Mark 12:24 by asking the rhetorical question, "Aren't you going astray?" After answering the specific question about marriage after resurrection, Christ returns to this idea of going astray here, pointing out that the real problem is that people don't understand the nature of life and death.
To understand Christ here, we have to understand a little about the Greek (and Roman) view of the afterlife and how it influenced the understanding of the resurrection in Christ's time. The Greek believed in an afterlife, but it wasn't a very pleasant vision. The dead were like wraiths, leading a half-life in the land of the dead. The god of the dead was Hades (Greek) or Pluto (Latin). This vision influenced the Jewish religious leaders of Christ's time. They either imagined that people lingered in this half-life of the dead (as the Greek saw it) or in the ground until the resurrection when they were reincarnated into physical bodies.
This means that, until the final judgment, the dead, including Abraham, Isaac and Jacob, were either ghosts or corpses. (The word that Christ uses for "dead" also means "corpse."
Christ clearly rejects this vision. I will leave it to each of my readers to say what his alternative might be. I will only suggest that either time (as we understand it) does not exist between our deaths, the final judgment, and resurrection or that immediately upon our deaths, we are given a new life but not the life in the body of resurrection, which we get at the final judgment. The later vision is supported by Christ's statement earlier about Elijah being reborn (Mat 17:12), but personally, I don't think we are meant to know what actually happens at death.
The joke here is about wandering a great distance, specifically the distances between life and death, but it is lost in the standard English translation. Christ even gets in a little alliteration to make his point even more amusing. The terms translated as "greatly err," which really means "wander" or "go astray" "a great way" is polus planao.
οὐκ (partic) "Not" is ou which is the negative adverb for facts and statements, negating both single words and sentences. The other negative adverb, μή applies to will and thought; οὐ denies, μή rejects; οὐ is absolute, μή relative; οὐ objective, μή subjective.
ἔστιν (verb 3rd sg pres ind act) "He is" is eimi, which means "to be", "to exist", "to be the case," of circumstance and events "to happen", and "is possible." (The future form is esomai. The 3rd person present indicative is "esti.")
πολὺ ( adj sg neut acc ) "Greatly" is polus, which means "many (in number)", "great (in size or power or worth)," and "large (of space)." As an adverb, it means "far", "very much", "a great way," and "long." -- The word translated as "many" means many in number, great in power or worth, and large in size.