You start with the uttlerly unshakable assumption that Jesus could not be wrong. Therefore, any time he says anything that's proven wrong, his words have to be poetically skewed in order for you to pretend he really meant something other than what he said. But if you've read the Gospels, you know deep down that Jesus was predicting an end within the time of his followers (a frequent gambit by religious con men, to inspire the imminent fear they need, among their followers, to exercise power). You also know that these prophecies ended up being incorrect.

It would be utterly imbecilic for anyone to think the word meant "humankind" rather than "generation," in this context, since "humankind" would be an absurd redundancy given what came before in the same passages. In Mark 13, Jesus rambles on and on about all the bad crap that'll happen before the end, including specific references to what the people of Judea will do, and he says "At that time men will see the Son of Man coming in clouds with great power and glory." It would be ridiculous for him to then follow up and say "mankind won't pass before these things will happen." By definition, mankind couldn't pass before men saw the Son of Man coming in the clouds, or there wouldn't be any men to see it. Therefore, only an imbecile could think that the word was meant to mean "mankind" rather than "generation." Only if it means "generation" is that sentence actually meaningful.

Here, a parable. Let's say I prophesize that "Obama will invade Israel, before the end of his term." Now let's say that his term ends and nothing like that has happened. Obviously, then, I'd be a false prophet. But a suitably gullible apologist could come along and say "well, 'term' in English doesn't just refer to a period of time in office, it can refer to any period of time, so really the prophecy meant that Obama would do this before the term of his life was over." The painfully obvious reply to this bit of apologetics is that "before the end of his term" would be hilariously pointless verbiage if it meant "during the course of his life," since it's definitionally true he will only be doing things while he's alive. In the exact same way, if that word meant "mankind" rather than "generation," it would be hilariously pointless verbiage, since the prophecy already definitionally required that mankind still be around. But, since so many people turn their brains off when it comes to religion, the obvious problem with the attempt at apologetics is ignored, in the context of Jesus' false prophecies.

And, of course, this false prophecy was just an expansion on a false prophecy that spans the breadth of that text and the other Gospels:

"There are some standing here that shall not taste death, till they see the kingdom of God."

"And he said to them, 'I tell you the truth, some who are standing here will not taste death before they see the kingdom of God come with power.'"

"Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom."

"But whenever they persecute you in one city, flee to the next; for truly I say to you, you will not finish going through the cities of Israel until the Son of Man comes. "

And there's no question that those who were alive at the time were very clear that Jesus meant the end would come in their own lifetimes. Otherwise, their words, drawing from that assumption, reported elsewhere in the Bible, would make no sense. Peter said, "But the end of all things is at hand: be ye therefore sober, and watch unto prayer." That's nonsensical prattling if he didn't mean that the end was truly at hand. He might have instead said, "be sober and pray, because in the afterlife you'll be punished or rewarded according to your acts." But he clearly was instead conveying that those hearing him were living in what he repeatedly called "the last times," and so, more so than even earlier generations, it was important that they be watchful, prayerful, and sober.

James said "Be ye also patient; stablish your hearts: for the coming of the Lord draweth nigh." Counseling people for patience based on the impending nature of any event only makes sense if you think it's actually coming at some point during the lifetimes of your listeners. I would say, "be patient, stock prices will bounce back" if I didn't think they'd bounce back in the lifetime of anyone in my audience.

Paul also thought he and many of his listeners would be alive at the end, "Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord." He didn't say, "those who are alive." He said, "we." In fact, he thought the time was so short that there wasn't even a point in getting married "... the time is short: it remainteth, that both they that have wives be as though they had none." He also thought that the things that were written were written for the benefit of those of his own time, who'd see the end of the world: "Now all these things happened unto them for examples: and they are written for our admonition, upon whom the ends of the world are come." He also told his followers that "....the God of peace shall bruise Satan under your feet shortly."

If you'd been present when Jesus said the things quoted in the Gospels (if, indeed, he said those things), you'd have come away with the impression that the end would come in your own lifetime, or at least the lifetime of many of those in the audience with you.... and that's exactly what Jesus meant to convey, like so many apocalyptic hucksters before and after him. Only because it's demonstrably true that the end didn't come in that time did apologists have to grasp at straws to come up with some other interpretation of his words.

Of course, Jesus' mistake about the timing of the end of the world is just the tip of the iceberg when it comes to false prophecies in the Bible. The Bible can't even get it straight whether the world is going to end or not. Ecclesiastes 1:4 says it never will.