There's good reason to question the assumption that prophecies like this should be taken literally, ie, literal human generations"But I tell you of a truth, there be some standing here, which shall not taste of death, till they see the kingdom of God"
If people can see "the kingdom of God" in their current, physical life then realised eschatology takes care of it
In this scheme, it never was about a predicted second coming, or the end of the age or anything apocalyptic
Now, obviously I know that the gospel context and Paul's writings and other NT letters contradict that interpretation i'm suggesting
But what I am suggesting also is that they are not reflecting the original realised context of the saying (it is a saying then, not a prophecy)
For evidence to support this, we have the gospel of Thomas where it's found in a realised context
So to sumarise, i think the original saying was turned into a prophecy by the gospel writers because certain sects of the early church expected an imminent apocalyptic Parousia of Christ in their life time, but others did not, and I think there's reasonable cause to think that originally this saying was simply about 'seeing' the kingdom of God in one's lifetime and not 'the end of the age'
It is only a failed prophecy if it was originally meant in the sense that it's assumed it has, but does it? I'm not convinced
Some of the Q reconstructions anticipate a whole load of apocalyptic prophecies, but there may be no such source closely tied to the parable sayings expected of Q also, just the doctrine of certain early sects seeping into the gospels as they were written