Prophesying is a difficult thing to do. The biggest problem we face when being told things by God is our interpretations of the things stated. We hear it with our thinking. I remember once being given a revelation where I was even taken out by Christ and shown the answer to a question I had asked. Yet upon returning I interpreted the information incorrectly in part, due to my ignorance in another area. It wasn't until about 8 years later that I realised my error.
I think the greatest classic case of this in Scripture was Caiaphas in regard Christ _
"And one of them, named Caiaphas, being the high priest that same year, said to them, You know nothing at all, Nor consider that it is expedient for us, that one man should die for the people, and that the whole nation doesn't perish. And this he said not of himself: but being high priest that year, he prophesied that Jesus should die for that nation; And not for that nation only, but that also he should gather together in one the children of God that were scattered abroad. Then from that day forth they took counsel together for to put him to death." John 11:49-53
So had we asked the prophet Caiaphas what the revelation had said, he would say that he was told Christ was to be killed for the sake of everyone; and it was their responsibility to have it done.
Joseph Smith took his revelation that if he lived to be 85 he would see Christ come, to mean that Christ would definitely come at that time, even if he didn't live to be 85.
Yet some prophesies are a case of, "if you do X, Y will happen." I have seen this to be true myself. When I haven't done X, Y didn't happen. However when I did X, Y did happen. The Scriptures contain many such prophesies in instruction format to us all. If we follow what God says certain blessings will come. The tithing and word of wisdom promises being examples of this (Mal 3:10-12, D&C 89:18-21). If we do wrong things then wrong things will happen (not that all wrong comes upon us from our mistakes, as we also suffer for the wrongs of society, the world, our families, friends etc). Yet these all fit in the same prophesy.
"Therefore, you son of man, say to the children of your people, The righteousness of the righteous will not deliver him in the day of his transgression: as for the wickedness of the wicked, he shall not fall thereby in the day that he turns from his wickedness; neither shall the righteous be able to live for his righteousness in the day that he sins. When I shall say to the righteous, that he shall surely live; if he trust to his own righteousness, and commit iniquity, all his righteousnesses shall not be remembered; but for his iniquity that he has committed, he shall die for it." Ezekiel 33:12-13
In other words if God makes a prophetic promise to a person (or for that matter a people) if they then turn away from him, the promise no longer stands.
And many prophesies Joseph Smith received were of this type that Ezekiel refers to. Promises were made when people were doing the right thing, yet as time progressed they turned away from God, and so the promise became null and void.
Many accusations have been thrown at Joseph Smith claiming that many of his revelations proved to be wrong. While it would be silly to claim that a man who had a prophetic calling thrust upon him with so little time to learn and no one to learn from, made no teething mistakes; let's examine some of these claims taken from Anti-LDS sites online.
"If Joseph Smith was a true prophet from God, then his prophecies should have come to pass. However, there are plenty of cases where they did not. For example, Joseph Smith claimed that before 1891, and before he reached the age of 85, Jesus Christ would return. Well, Joseph Smith did not reach the age of 85 - he was murdered on the 27th of July 1844. And we know that Jesus Christ didn't return before 1891, bringing an end to the present world order."
The statement that Joseph received said that IF he lived to be 85 he would see Christ coming (D&C 130:15). This means that had events been such that Joseph Smith stayed away rather than going back to be murdered his continued life would have changed events that significantly.
"Smith also predicted the demise of the government of the state of Missouri which had displeased him - this never happened." He prophesied that the civil war would not result in the freedom of 'Ham' - this could not happen until the millenium according to Smith. Here again Smith was demonstrating just what a false prophet he was."
As to the then government of Missouri. That political party lost the next election and dissolved soon afterward.
In regard Ham's descendants (which were pre-flood Canaanites) the Bible tells us _
"And he said cursed be Canaan; a servant of servants shall he be to his brothers. And he said, Blessed be the Lord God of Shem; and Canaan shall be his servant. God shall enlarge Japheth, and he shall swell in the tents of Shem and Canaan shall be his servant." Gen 9:25-27
Note it mentions that of the 3 sons of Noah only Shem and Japheth will have Canaan as servants. So who was Canaan? Only Ham is left of the 3 sons. And why call him Canaan since he was their brother? Surely he would have his children as the same descent as his brothers, wouldn't he? Obviously not. Ham must therefore have married a Canaanite. and why was her race predominant?
And who have been servants more than others of races upon the earth? The negros, is the answer.
Are the negros no longer slaves? Did the Civil war end the slavery of even the negros in the USA? I would argue that negros today, while having much greater freedom generally, are still not in equal situation to whites completely. So I would see that the prophesy of Joseph Smith and Genesis chapter 9 is still valid.
"Smith prophesied in 1832 concerning the American civil war that the British would get involved and the war would extend to all nations. This is just one of his false prophecies concerning the American civil war."
As to the British getting involved in the Civil war. D&C 87:3 doesn't state that Britain will come to the aid of the confederacy. It states that they would be called upon to do so. And they were so called upon. Prophesies must be examined very carefully WITHOUT additions.
It then states that Britain would do the same, as would other nations. This is presenting a tendency toward world wars and large unions would then arise. This happened only around 50 years after the conclusion of the Civil War in 1914. Then only 31 years later we had wars that put almost everyone on earth at war in 1939. Nations were calling upon nations to assist and forming large alliances. Just over 50 years after the Civil War ended "the League of Nations" was formed. After this came the United Nations that we have today. This wasn't done before on anything like the present scale.
Then in verse 4 it goes on to mention the coming problems which aren't upon us just yet (enjoy the peace). Then in verse 5 he mentions the rising of the Lamanites (across the Americas) against the Gentiles, as is prophesied in the Book of Mormon. Then in verse 6 .... well... pray hard and keep close to the Lord.
It should also be remembered that in this prophesy he has predicted the Civil War, what state it would begin in and what it was about.
It is very important to read prophesies carefully rather than a quick scanning. Listenning to prophesy correctly is an art form where we learn from our mistakes. After enough errors we wake up to stop adding our own bits to it. Just listen to the instruction and take it on face value.