Jesus says in three of the Gospel accounts (Matthew 16:28, Mark 9:1, Luke 9:27):
Mark 9:1
1 And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.
Critics of the Bible have construed this a failed prophecy, implying that Jesus was saying that His second coming would take place within the disciple's lifetimes.
This prophecy is not referring to the Second Coming of Christ at all. It's referring to the event called the Mount of Transfiguration.
In all of the accounts, just after the passage in which Christ gives the prophecy, we are immediately taken to the scene where Christ is transfigured into His glorified form, and Moses and Elijah appear:
Mark 9:1-4
1 And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.
2 And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.
3 And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.
4 And there appeared unto them Elias with Moses: and they were talking with Jesus.
In the above passage, I think the meaning of verse 1 is obvious to anyone who reads even one more verse. Quoting verse 1 out of context as an unfulfilled prophecy is dishonest. Once again, the immediate jump to the account of the Mount of Transfiguration is the same in all three accounts (Matthew 16:28-17:2, Mark 9:1-4, Luke 9:27-29).
This is not a failed prophecy, and portraying it as such by quoting it piecemeal from the context is dishonest.