Should I give a talk about an insignificant result?
I'm pretty sure you aren't alone here. People find surprising things all the time. Be thankful that you are the one who found it. 

This may sound risky, but I don't believe it is. Give a technical description of the problem as you normally would and then give an historical tour through what you originally thought and what you now think and why. 

Rather than being career suicide, you will, IMO, be thought refreshingly honest and open to the truth wherever it leads. If you "get away with it now" but don't later, things will be much worse. Much much worse is if a student in your talk raises his/her hand with ... well, you know. 

Out with it my friend. Just take a deep breath and say it all. You can even say that you feel a bit silly now that you've gotten a deeper understanding. Noting wrong about a deep understanding, of course, even if it eludes you early. 

And, of course, your more senior colleagues and the reviewers, if any, also missed the point. You are a hero, not a bum. The math is what it is. 



Another option, of course, is to say that you have found an issue along with the consequence but then talk about another topic. But I think that would disappoint people. Seeing the flaw can, itself, be enlightening.