Jesus predicted that on his return to earth, "the sun will be darkened, and the moon will not give its light, and the stars will be falling from heaven, and the powers in the heavens will be shaken. And then they will see the Son of man coming in clouds with great power and glory. And then he will send out the angels, and gather his elect from the four winds, from the ends of the earth to the ends of heaven." (Mark 13:24-27) He even predicted a deadline for it to happen: "Truly, I say to you, this generation will not pass away before all these things take place." (Mark 13:30) The generation he was speaking to did pass away, but the sun still shines, the stars still twinkle in the night sky, and there have been no heavenly passengers riding in on the clouds. Jesus was wrong.
Mark 13:30 is not the only passage that makes such a prediction. We see it also in Matthew 24:34, which uses the same language as Mark. Luke 21:25-27, 32 also has nearly the same wording.
Also, Matthew 16:28 – "There are some standing here who will not taste death before they see the Son of man coming in his kingdom."
And in Luke 9:26-27 – "There are some standing here who will not taste death before they see the kingdom of God."
And Mark 9:1 – "Truly I say to you, there are some standing here who will not taste death before they see that the kingdom of God has come with power."
The Greek word for "generation" means essentially the same as our English word. According to the New Oxford Annotated Bible (1973 ed., p. 1204, note to Matthew 24:34), "the normal meaning of this generation would be ‘men of our time,’ and the words would refer to a period of 20-30 years."
The theologians often appeal to Mark 13:32 to argue that Jesus was not making a specific prediction about when the end would occur. That passage tells us that Jesus himself does not know exactly when the end will come: "But of that day or that hour no one knows, not even the angels in heaven, nor the Son, but only the Father." Here, however, he’s just telling us that he doesn’t know the exact time when the kingdom of God will arrive. He is still very clear that it will occur sometime during the lifetime of the generation he’s speaking to.
Another way in which the theologians try to escape from this embarrassment is to claim that the generation referred to in Mark 13:30, Matthew 24:34, etc. is not the generation Jesus was speaking to, but the generation that will be living at the time these amazing events take place. But this is obviously absurd, because it would mean that Jesus is telling his audience, "Some of the people in the generation that will be alive when these things happen will be alive when these things happen." A statement like this conveys no meaning at all, and there would be no point in Jesus saying such a thing.