The Oracle has the ability to make Prophecies. This is not the same as prediction or even highly accurate prediction. Prophecy, as we choose to define it, is an infallible statement about the future. This implies that there is one and only one true timeline. When a Prophecy "fails" it is usually because of interpretation of its meaning much as Neo misunderstood what The Oracle told him. There's is resistence to the idea that Fate exists and that there is only one line of events, because, as humans we have the ability to see (or predict) alternative futures. We like to control our own lives and always wonder "What if...?" but The Oracle is a totally separate being in that regard. The vase comment was a joke at Neo's expense, much like the "But not too bright" comment. In short, The Oracle is always right. Neo is told, however, not to think in those terms because interpretation and perspective matter a lot in understanding Prophecy, but in the big picture, The Oracle is infallible.
How is this possible? We say it is because the Matrix is deterministic. There is one and only one line of events inside the Matrix where quantum mechanics do not exist. So, The Oracle, herself is bound by this constraint when she speaks (we are too, but cannot see it from our perspective) or acts. For her, there really is no "If" except when speaking from a human predictive prespective (showing options).