In Lost season 3, when Desmond tells Charlie about another vision where Charlie drowns,
he tells him that he also saw Claire boarding the helicopter with the baby. As we see later, Claire doesn't leave the island and Kate takes Aaron with her. Moreover, the baby has already grown up to a kid, so I suppose Desmond would have mentioned Aaron's approximate age if that was the case. (If that is still bound to happen).
So did Desmond lie to him in order to fulfill something else he actually saw there and wanted to happen?
I currently am watching season 4, so if that is explained later (which I doubt because of the last sentence under spoiler tag), I'll take that as an answer to avoid spoilers.