The following is an excerpt from Dr. Thomas Holland's Crowned With Glory, ©2000, used with permission.
Mark 16:9-20 (The last twelve verses of Mark)
This passage is referred to as the "Longer Ending" of Mark. Many textual critics doubt its authenticity, believing it was an addition made in the second century. It often appears in modern versions in brackets with footnotes questioning its authenticity.  Most textual scholars believe that the text abruptly ends after verse eight. Even the so-called "Shorter Ending" that is added after verse eight is considered to have originated in the second century. The shorter ending reads:
But they reported briefly to Peter and those with him all that they had been bold. And after this, Jesus himself sent out by means of them, from east to west, the sacred and imperishable proclamation of eternal salvation. 
Most scholars believe the original ending to Mark's Gospel has been lost.  If this is true, the concept of preserving the words of Scripture is forever annihilated. The words cannot be preserved and lost at the same time. However, textual scholars usually call for its inclusion even if they question its originality. Dr. Bruce Metzger departs from the maxim of modern textual critics, Brevior lectio potior (the shorter reading is preferable), and supports the longer ending even though admittedly he does not regard the passage as genuine. He considers it to be a legitimate part of the New Testament because of its traditional significance to the body of Christendom.  The passage is not contained in the Alexandrian texts, minuscule 2386, the Syrian Sinaitic Version, and a few other translations.
However, it is in many of the Greek uncials (A, C, D, K, X, D, Q, and P) dating between the fifth and ninth centuries. It is also contained in later dated Greek minuscules (137, 138, 1110, 1210, 1215, 1216, 1217, 1221, and 1582). It is the reading found in the majority of Old Latin texts as well as the Coptic versions and other early translations. Finally, it is cited (at least in part) by many of the early church fathers such as Justin (165 AD), Tertullian (220 AD), Hippolytus (235 AD), Ambrose (397 AD) and Augustine (430 AD). 
In 177 AD Irenaeus wrote Against Heresies. In it he cites from Mark 16:19, establishing that the longer reading was in existence at this time and was considered canonical, at least by Irenaeus:
Also, towards the conclusion of his Gospel, Mark says: "So then, after the Lord Jesus had spoken to them, He was received up into heaven, and sitteth on the right hand of God; " confirming what had been spoken by the prophet: "The LORD said to my Lord, Sit Thou on My right hand, until I make Thy foes Thy footstool." Thus God and the Father are truly one and the same; He who was announced by the prophets, and handed down by the true Gospel; whom we Christians worship and love with the whole heart, as the Maker of heaven and earth, and of all things therein. (3:10:5).
The difference here is extremely important. If we conclude that this passage is not authentic, then we must question what happened to the original ending of Mark. It is not logical that the Gospel would end at this place so abruptly. Nor is it likely, as some scholars have suggested, that the Gospel was never finished, calling biblical inspiration into question. The conclusion held by most textual scholars, whether liberal or conservative, that the original ending has been lost over the passage of time certainly denies the doctrine of biblical preservation. If we allow that a passage of inspired Scripture has been lost from this section of the Bible, what stops us from making the same application to other passages? It is certainly within the realm of scholastic studies to note any and all textual differences. But once we open the possibility that this or that passage has been lost, we are now trusting in the understanding of men over the biblical promises of God. Certainly it is better to embrace the textual evidence and hold to the promise of preservation.
 Kurt and Barbara Aland, The Text Of The New Testament (Grand Rapids: Eerdmans, 1989), 232.
 Revised Standard Version, footnote
 Bruce Metzger, A Textual Commentary On The Greek New Testament, 105.
 Bruce Metzger, Christian History (interview with Dr. Metzger downloaded from Christian History Magazine, America Online, 9/17/96).
 John William Burgon, The Revision Revised (Paradise, PA: Conservative Classics, 1883), 422-423. Burgon also supplies additional names of church fathers who support the reading.