This is can be a very misleading cliché, because there truly are opportunities you will never have again in your lifetime.  Going out with Mary Ann Joblonski was a once in a lifetime opportunity, and it’s never going to happen again.  Even if I went out with her the first time, and she’s willing to go out with me again, the first time is only going to happen once.  And that’s because everything is a once in a lifetime opportunity.  That’s why companies can advertise that their sale is a “once in a lifetime opportunity!”  Even if they put the exact same thing on sale again next week for the exact same price, it’s still the only opportunity you’re ever going to have to buy it for that price right now.  And you wouldn’t want to miss that, would you?