Would it mean that the second coming of Christ actually happened at the Pentecost when Jesus brought the Holy Spirit? The only one who wasn't alive was Judas Iscariot as to the meaning as "some of you". Does this make sense?
Um....that wouldn't be my take on it, no. Matthew 16: 27-28 says: For the Son of man is destined to come in the glory of his Father with his angels, and then he will recompense each one according to his behavior. Truly I say to YOU that there are some of those standing here that will not taste death at all until first they see the Son of man coming in his kingdom.”
The very next chapter (Matthew 17) and verse (1) says: "...Six days later Jesus took Peter and James and John his brother along and brought them up into a lofty mountain by themselves. And he was transfigured before them, and his face shone as the sun, and his outer garments became brilliant as the light. And, look! there appeared to them Moses and E·li´jah, conversing with him..."
What I get out of this is that Jesus was simply showing them how he would return at some point in the future. While the events of Pentacost were significant, there wasn't anything that resembled Jesus returning with kingly power. It would happen at an unspecified future date.
I could be wrong, but that's my interpretation of it.