Source: Benatar, David. Better Never to Have Been (2008 1 edn). pp. 58-59.
Now it may be objected that one cannot possibly be mistaken about whether one’s existence is preferable to non-existence. It might be said that just as one cannot be mistaken about whether one is in pain, one cannot be mistaken about whether one is glad to have been born. Thus if ‘I am glad to have been born’, a proposition to which many people would assent, is equivalent to ‘It is better that I came into existence’, then one cannot be mistaken about whether existence is better than non-existence. The problem with this line of reasoning is that these two propositions are not equivalent. Even if one cannot be mistaken about whether one currently is glad to have been born, it does not follow that one cannot be mistaken about whether it is better that one came into
existence. We can imagine somebody being glad, at one stage in his life, that he came to be, and then (or earlier), perhaps in the midst of extreme agony, regretting his having come into existence. Now it cannot be the case that (all things considered) it is both better to have come into existence and better never to have come into existence. [1.] But that is exactly what we would have to say in such a case, if it were true that being glad or unhappy about having come into existence were equivalent to its actually being better or worse that one came into being. [2.] This is true even in those cases in which people do not change their minds about whether they are happy to have been born. Why so few people do change their minds is explained, at least in part, by the unduly rosy picture most people have about the quality of their own lives. In the coming chapter, I show that (with the exception of real pessimists, who may have an accurate view of how bad their lives are) people’s lives are much worse than they think.
Assume 1: being glad/unhappy that one was born = actually being better or worse that one was born.
Then even if someone's own live doesn't warrant optimism, how's 2 true? I interpret 2 to signify: everlasting happiness about one's birth can still imply being mistaken on whether one was born.