A quotation from the website:
In other words, not every word a prophet says is always considered prophesy. In addition, some prophesies are conditional in nature. For example, in the Holy Bible, the prophet Jonah prophesied that Ninevah would by overthrown within forty days:
“And Jonah began to enter into the city a day’s journey, and he cried, and said, Yet forty days, and Nineveh shall be overthrown.” (Jonah 3:4)
However, the people of Ninevah repented and the Lord withdrew the prophecy:
“And God saw their works, that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did [it] not.” (Jonah 3:10)
Was Jonah a false prophet? No, the prophecy was simply a conditional one.
It’s rather common for anti-Mormons to try and turn every single statement ever said by any Church leader at any time into an official prophecy that has no conditions. What’s especially hypocritical about this technique is that they are using the same techniques that atheists use against them. If you want a good response as to why Fundamentalist Protestant attacks against LDS Church leaders are invalid, I suggest you go to a Fundamentalist Christian bookstore, buy a book written by a Fundamentalist Christian author that refutes the attacks of atheists against the Bible, and simply apply the same principles to LDS Church leaders (e.g., "Jonah wasn’t a false prophet because the prophesy was conditional in nature. Likewise Joseph Smith wasn’t a false prophet because the prophecy was conditional as well)
lightplanet.com/mormons/response/qa/false_prophesies.htm