In Kings, the prophet Elijah is portrayed ascending to heaven. This is 2 Kings 2:11:
Jesus in the gospel of John seems to say that he is the only one who ever ascended into Heaven. This is John 3:11-13:
There are various solutions to this problem, most of them unlikely. But I think the most plausible solution is that Jesus is just being vague. His statement that nobody has ascended to heaven has an exception, but he may simply have felt the exception was not relevant to the discussion. His primary message here is that nobody around him has more authority than he does. This is fairly close to the response Looking Unto Jesus gives, though I don't find it totally convincing.
Tektonics' solution is that Elijah was merely taken up into "the heavens" (meaning the sky) rather than Heaven with a capital H, but this makes little sense. What exactly was Elijah doing in the sky?
Dan of Israel suggests that Jesus is talking about ascent into heaven under one's own power. Thus, "he ascended into heaven" is different from "he was taken into heaven by God". But this doesn't look like what Jesus means to me.
For a non-infallibilist, of course, the most likely explanation is simple error, either on the part of Jesus or John.