Although it's true that the first Christians did believe that the Second Coming was imminent, Christ also taught that "you do not know the hour".
Mt 24:44 — Therefore you also must be ready; for the Son of man is coming at an hour you do not expect.
Mk 13:32–33 — But of that day or that hour no one knows, not even the angels in heaven, nor the Son, but only the Father. Take heed, watch and pray; for you do not know when the time will come.
Lk 12:40 — You also must be ready; for the Son of man is coming at an hour you do not expect.
As the question recognises, this hasn't happened yet. Thus it's easy to see that the belief that the Second Coming was imminent was a misunderstanding — not that it's a bad thing to live each day as if it were your last because you might face the final judgement tomorrow. That's still the case (cf Jas 4:13–15).
So the question becomes one of interpreting Mt 16:28: "Truly, I say to you, there are some standing here who will not taste death before they see the Son of man coming in his kingdom," which is also recorded in Mk 9:1: "And he said to them, 'Truly, I say to you, there are some standing here who will not taste death before they see the kingdom of God come with power.'"
If we are to believe that Christ did not lie to his hearers, or was mistaken himself, then a plain reading of these verses as referring to the Second Coming is obviously wrong: those standing there have certainly died by now.
There are two potential interpretations.
One is that "seeing the kingdom of God come with power" refers to Jesus' own death, when "the curtain of the temple was torn in two, from top to bottom; and the earth shook, and the rocks were split; the tombs also were opened, and many bodies of the saints who had fallen asleep were raised" (Mt 27:51–52). This is by far the easiest interpretation, as it admits a plain reading of the scripture. The kingdom of God had shown its power, and they would have seen it.
The second, deeper, interpretation is that the death Jesus was referring to was the everlasting damnation to Hell.
Each man receives his eternal retribution in his immortal soul at the very moment of his death, in a particular judgement that refers his life to Christ: either entrance into the blessedness of heaven—through a purification592 or immediately,593—or immediate and everlasting damnation.594
At the evening of life, we shall be judged on our love.595
Catechism of the Catholic Church
In this interpretation, experiencing death before seeing the power of God needs explanation.
Jesus' use of before was to remind his hearers that the judgement could come at any time, and they should live each day as if it were their last. For if the Second Coming were tomorrow, they would not die before seeing it: they might see it and die tomorrow, and they would be judged tomorrow. In order that the judgement tomorrow would not condemn them, they needed to conform their lives to his teachings.
The affirmations of Sacred Scripture and the teachings of the Church on the subject of hell are a call to the responsibility incumbent upon man to make use of his freedom in view of his eternal destiny. They are at the same time an urgent call to conversion: "Enter by the narrow gate; for the gate is wide and the way is easy, that leads to destruction, and those who enter by it are many. For the gate is narrow and the way is hard, that leads to life, and those who find it are few."616
Since we know neither the day nor the hour, we should follow the advice of the Lord and watch constantly so that, when the single course of our earthly life is completed, we may merit to enter with him into the marriage feast and be numbered among the blessed, and not, like the wicked and slothful servants, be ordered to depart into the eternal fire, into the outer darkness where "men will weep and gnash their teeth."617
CCC
This interpretation takes Jesus' teaching out of his own time and its narrow application to just his own contemporaries, and allows the Eternal God to apply it to all time, including to hearers today.
592: Cf. Council of Lyons II (1274): DS 857-858; Council of Florence (1439): DS 1304- 1306; Council of Trent (1563): DS 1820
593: Cf. Benedict XII, Benedictus Deus (1336): DS 1000-1001; John XXII, Ne
super his (1334): DS 990
594: Cf. Benedict XII, Benedictus Deus (1336): DS 1002
595 St. John of the Cross, Dichos 64
616: Mt 7:13-14
617: Lumen Gentium 48 # 3; Mt 22:13; cf. Heb 9:27; Mt 25:13, 26, 30, 31 46
