I doubt that Jesus lied or made a mistake. It is probable that we just don't have the story straight.
I've dug through lots of ideological disagreements. Fortunate is the man whose denomination tells him how to interpret something like this.
Around 2001, I desperately wanted to go to Bible College and was greatly dismayed to find that some of the courses were a sort of review of hundreds of sermons that you would be expected to use when preaching. No original thought there.
Also, the Shia Muslims believe there were 12 Imams (Pastor, or Rabbi) and that one of them is still alive and waiting for the return of Isa PBUH (Jesus) to help him to make sense of the mess here. So maybe in the NT, Jesus was just saying that some people from that generation would survive until his return? I think I may have heard this preached in one of those off beat Christian Churches ???