1 Samuel 21:8-9 NASB
 David said to Ahimelech, "Now is there not a spear or a sword on hand? For I brought neither my sword nor my weapons with me, because the king's matter was urgent."
Then the priest said, "The sword of Goliath the Philistine, whom you killed in the valley of Elah, behold, it is wrapped in a cloth behind the ephod; if you would take it for yourself, take it. For there is no other except it here." And David said, "There is none like it; give it to me
But wait, as you keep reading one day you come to 2 Samuel 21:19 which says...
2 Samuel 21:19 NASB
 There was war with the Philistines again at Gob, and Elhanan the son of Jaare-oregim the Bethlehemite killed Goliath the Gittite, the shaft of whose spear was like a weaver's beam.
What? Elhanan killed Goliath? I thought David killed him? Who is Elhanan? Was there another giant named Goliath? Was David's middle name Elhanan? Why does my bible contradict itself? And if it has mistakes how am I supposed to trust it? And that is exactly the point. Modern translations like the NASB are not trustworthy. They have been translated from faulty manuscripts, namely the 1881 revision of the Greek NT published by Westcott and Hort which has changed over 60,000 words and omitted many entire or partial verses and some paragraphs. This is serious! And if you had been reading a King James bible you would know that Elhanan killed "the brother of Goliath of Gath". But the new versions like the NASB, NIV, HCSB, ESV etc...they took that part out.
One motivation for the revision and modern translations was in order to put a copyright on the Bible because greedy men wanted to make money from the sale of the bestselling book of all time. But that is just on the surface. The underlying plan of Satan is to wreck people's faith and thereby their lives also. The men involved in the 1881 revision were only pretending to be Christian. They were heavily involved in the occult and by their own admission in their writings, the literature they most esteemed was not the word of God but rather the works of Plato and Aristotle and other pagan philosophers. They also admit in their own writings that they were concocting heresy with the revisions they were making to the Greek text and Hort was very vocal about his contempt for the Textus receptus manuscripts that the KJV was translated from.
Up until the 1880's the KJV was the standard English bible and was received as the Holy word of God by all who love Him. It has a 400 year track record and was translated by 54 God fearing men who felt they were called to a mission of preserving the word of God, and were appointed by King James, men who spent seven years painstakingly translating word for word the best manuscripts. You can read more about them here av1611.com and many other places, do a Google search or I also recommend a book by G.A. Riplinger titled " In Awe of Thy Word" which goes into detail about these men and has much more content that will bless you! These men were of such integrity that when they had to add a word to make a sentence flow in English they put that word in italics so the reader would know it was not in the Greek or Hebrew text.
I challenge anyone reading this who has questions about the difference between bible versions not to take my word for it... you compare them for yourself and see what you find. Please do it! Don't let the modern versions that have contradictions lead you to a place where you feel you can't trust the word of God! There is a bible you can trust and it's been trusted and proven for centuries!
Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth. 2Timothy_2:15 KJV
May the Lord Jesus bless you as you search out this matter. Amen!