According to many ex-Christians, one of the things that makes them think that Jesus was just a loon and not the true messiah/god was that his prophecies in Mark never came true. One of the lesser pointed out false prophecies of Jesus comes from his trial, where he says:
60 Then the high priest stood up before them and asked Jesus, “Are you not going to answer? What is this testimony that these men are bringing against you?”
61 But Jesus remained silent and gave no answer. Again the high priest asked him, “Are you the Christ, the Son of the Blessed One?”
62 “I am,” said Jesus. “And you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven.”
Obviously, the high priest here did not live to see Jesus (or the Son of Man) descend from the clouds sitting at the right hand of power (or the mighty one). So this is an unambiguously failed prophecy. These sort of failed prophecies are littered all throughout Mark. He has Jesus predicting the end of the world within his generation, or telling the people around him that they would not die until they witnessed the Son of Man and/or the kingdom of god arrive on Earth.
Now what about the criterion of embarrassment? This is the argument that Christians would not have maintained or invented traditions that embarrassed them. Thus, any failed prophecies would not have stayed alive within the tradition or would not have made it to Mark's gospel, or they would have been rewritten/reinterpreted so that they could be “fulfilled” in a more spiritual sense (i.e. later Christian writings like Luke 17.21 or John 18.36). So why would Mark write an entire gospel filled with prophecies that failed once Jesus' generation was dead? Mark must have thought that those prophecies hadn't been fulfilled yet, thus he must have seen himself as part of the generation that those prophecies applied to.
There's no indication that Mark is trying to pretty up or spiritualize the prophecies of Jesus like Luke and John do. So Mark most definitely thought that these prophecies applied to his own generation. But the question now becomes: Was Mark apart of Jesus' generation? If Mark was written in 40 CE then I could see these definitely being the words of Jesus, since Mark and Jesus would be in the same generation. But most scholars conclude that Mark was written literally a generation or two later, when the people who would have heard Jesus preach were more than likely dead. In my estimation, Mark was written by someone who had not yet been born or was very young when the 2nd temple was destroyed in 70 CE, as this accounts for the many post 70 CE anachronisms in Mark. So I would put the writing of Mark circa 80 – 90 CE.
In 80 or 90 CE, any prophecy about people in 33 CE not dying until the Son of Man came swooping down from heaven on a cloud ushering the new kingdom of god would quite obviously be false (especially with a large war in between killing many in that prior generation), and no one around that time period would write something like that per the criterion of embarrassment. But it's obvious that Mark was not embarrassed by those prophecies so he “kept” those prophecies in, so the only reasonable conclusion I can make is that Mark really did think that those prophecies applied to his own generation. And that it is Mark himself who was the apocalypticist, not Jesus. Mark just used Jesus as his sockpuppet to make prophecies that he thought applied to his own generation. Thus he has Jesus say “let the reader understand” (Mk 13.14).
Finally, this in no means forces the conclusion that Jesus did not exist. It just means that an itinerant preacher Jesus who preached the end of the world wasn't in the mind of the pre-Markan Christians. That model of Jesus is more than likely the creation of Mark himself. This also makes sense of the silence of pre-Markan Christian writings on the teachings of Jesus. There were none. Unabashedly, this still fits with my hunch about who the historical Jesus was.