"Worst possible" might be a bit strong, but an important property that a voting system should have is that it finds the Condorcet winner if one exists. The Condorcet winner is the candidate who beats every other candidate in a head-to-head election. IRV is not guaranteed to find a Condorcet winner.
The problem with IRV is that you can have a candidate who is everyone's second choice, but then loses out on the first ballot because too few people ranked them first.
I've been voting RCV and I've yet to see this actually occur in practice. The candidates who are marketing themselves so well that they are second on almost everyone's ballot (in a 13 person race!) are generally FIRST on a fair number (relative to all 13 folks).
The problem with IRV is that you can have a candidate who is everyone's second choice, but then loses out on the first ballot because too few people ranked them first.