The King James Version is one of the most respected and admired translations of the Bible in all of history and is still very popular today. While it actually has a lot of beautiful English in it, it should not be thought of the perfect and only true translation the way many Protestants (even today) think. These Protestants are known as King James Onlyists. One very good, quick, and easy traditional argument for discrediting the "flawlessness" of this translation is to point to 1 Corinthians 11:27, where Paul speaks about the sin of abusing the Eucharist:
KJV: Therefore whosoever shall eat this bread AND [Greek:ἤ] drink this cup of the Lord unworthily shall be guilty of the body AND [Greek:καί] blood of the Lord.
The KJV has botched a key word here by using the Greek word "AND" in both places when in fact the Greek word "AND" (kai) only appears in the second instance. In the first instance, the Greek word is actually "OR". How it should read is: Abusing EITHER the consecrated bread OR the consecrated cup makes one guilty of BOTH the body AND blood of Jesus. You might think, what's the big deal? The big deal is, this text is powerful for demonstrating the Catholic teaching that Jesus is fully present under either bread alone or cup alone, something Protestants repudiate since this only makes sense with transubstantiation. The KJV wants to get around this by tampering with the text and adding "AND" so that the text denies Jesus is fully present under each, and that each must be abused to be guilty of both.
Conclusion: This is not to say the KJV is a horrible translation, but only that it is not flawless and has biases that affect key doctrine that cannot be swept under the rug. This should be simple enough to memorize that any Catholic should be able to pull this out when needed, even to Protestants who are not KJV Onlyists. Lastly, not only does the Greek totally refute the KJV here, but no respected Protestant translation uses "AND" here, they all use "OR" (see the list HERE), confirming that this isn't a Catholic invention but being true to the Greek.