No offense. This is only expressing my view on the subject. It is by no means the correct explanation. Even physicists fail to reach a unanimous conclusion.
Can fortune telling be 100% accurate? I personally don't think so.
Take astrology as an example. There is a similar discussion in the classic novel, Sophie's World. Astrology studies the location of other stars and planets that can be millions of light-years away from us (light-year is the distance that light can travel in one year). Therefore, what we are observing on Earth is actually the light that came from the stars and planets millions of years ago. These stellar objects are very likely to be in a different position now, or might even no longer exist. If you still think astrology is correct, then you are probably supporting predictive determinism, i.e. everything, including why you and I exist and what you will be doing at this time tomorrow, is pre-determined by other events that are measurable (even though they happened ages ago).
Heisenburg Uncertainty Principle refutes predictive determinism. The principle states that one cannot measure the position of an electron precisely without disturbing it. There is no way to measure things 100% accurately, and thus no way to construct a function that forecasts future events 100% accurately. Every attempt to predict the future, be it astrology or tarot cards or Chinese palm reading, is deemed to be error-prone.
(Side note: This, however, does not refute casual determinism. It is still possible that everything is pre-determined. It is just that we cannot measure the events that determine it. Albert Einstein is a believer in casual determinism. He once commented, "God does not play dice with the universe," referring to his view that things do not happen in an entirely random fashion.)
Practically, there are often fortune tellers that have made every prediction correct. This, however, can be a statistical phenomenon.
Suppose there is a coin-flipping competition with 1000 participants. In every round a participant is eliminated if he flips a tail. On average, half of the participants are eliminated in each round. Suppose that there is a final "winner" who flips 10 consecutive heads, and all the other players have flipped a tail at some point. While some might attribute his winning to his superior skills, he achieves this only by chance (the chance of flipping 10 heads is 1/1024, so among 1000 players there is about 1 person who is able to do so). If he is asked to make the 11th flip, he still has 1/2 chance of getting a tail, like an average person does.
The moral of this story is: even if no one has superior forecasting ability, some fortune tellers can still seemingly outperform others and gain a 100% track record. With so many fortune tellers in the world, there ought to be some winners. However, if they are asked to forecast one more event, they might not be outperforming average fortune tellers, since they achieve the record only by pure luck.
