In the King James Bible, 1 Peter 4: 8 reads, "And above all things have fervent charity among yourselves: for charity shall cover the multitude of sins."
The Joseph Smith translation of this verse, probably from sometime between 1830 and 1833, modifies the last clause to, "for charity preventeth a multitude of sins."
The difference in meaning seems significant: covering is retroactive (you can only "cover" after a sin is committed), whereas preventing is proactive (you can only "prevent" before a sin is committed). The shift from the KJV to the JST relocates the scriptural principle in terms of time. That matters--right?
Here's Joseph Smith's most famous use of this scripture, from comments made on 7 November 1841: "If you do not accuse each other, God will not accuse you. [...]If you will throw a cloak of charity over my sins, I will over yours—for charity covereth a multitude of sins." (See History of the Church, 4: 445)
Notice that Joseph uses the King James wording rather than quoting from his own translation. Consequently, the quote's meaning seems far more closely related to the doctrine expressed in the King James version than that expressed in the JST. Why would Joseph do that?
If the JST consists of necessary corrections from inaccurate doctrines (due to mistranslation) to accurate ones (expressed in the new wording), then that famous 1841 quote is doctrinally compromised. If, on the other hand, the 1841 statement, representing "newer" revelation, replaces the "older" translation of the verse (probably from 1830-1833), then the JST markings in our scriptures may not be terribly reliable. The underlying question is whether it's better to read the JST as ruling out the old meaning of a passage knowing that it will also rule out future uses of the old wording, or better to let new use of old passages lead us to dismiss portions of the JST.
Perhaps, before choosing one of the above alternatives, we could consider a possibility that doesn't give in as easily to such either/or thinking:
What if the Joseph Smith translation is not typically designed to replace one meaning with another, but to suggest a richer range of meanings of a passage than can be expressed by a single word in English? In English, for example, "cover" and "prevent," as described in this post, speak to different parts of time relative to the moment of sin. But what if God, who has all time continually before him, speaks a native language that doesn't distinguish between these two? What if, for God, one word encompasses both "cover" and "prevent"? The JST phrasing, then, would not be superior but rather supplementary to the King James phrasing, each hinting at a different aspect of the intent of the text (which, according to this view, cannot be properly translated into a single verb in English).
Would we understand the gospel better if we believed that what God can say in one word often takes at least two apparently contradictory human words to express?