Hello,
"Verily I say unto you, there are some of them that stand here, who shall in no wise taste of death, till they see the Son of man coming in his kingdom."Matthew 16:28
"And after six days Jesus taketh with him Peter, and James, and John his brother, and bringeth them up into a high mountain apart:and he was transfigured before them; and his face did shine as the sun, and his garments became white as the light." Matthew 17:,2
Peter, James and John saw a miniature of the second coming before they die.
It is significant that Matthew, Mark, and Luke record the narrative of the Transfiguration immediately following this prediction. There is no break in the narrative--no chapter or verse division in the Greek original--and furthermore all three mention the fact that the Transfiguration occurred about a week after this statement, implying that the event was the fulfillment of the prediction. The connection between the two sections of narrative seems to cancel the possibility that Jesus here referred to anything but the Transfiguration, which was a miniature demonstration of the kingdom of glory. Without a doubt, Peter also understood it like explained, check 2 Peter 1:16-18.
"...but we were eyewitnesses of his majesty. For he received from God the Father honor and glory, when there was borne such a voice to him by the Majestic Glory, This is my beloved Son, in whom I am well pleased"