Most modern translations include what is called the "long ending" to the Gospel of Mark. The problem is that this ending was never in the original manuscripts. In fact, this ending isn't even in the surviving manuscripts from several centuries after the autograph. (See Codex Sinaiticus, c. AD 320)
Early Church leaders were troubled by what they saw as an abrupt ending to Mark. In it, the ladies are going to the tomb to anoint Jesus' body with oil, but they arrive to find the boulder rolled away and the tomb empty. They are told by an unidentified young man in a white robe that Jesus has risen and that they are to tell the Apostles. The final verse ends with them being afraid.
How did it originate?
The modern long ending borrows some of its text from the Great Commission at the end of the Gospel of Matthew, but other text is completely new. There are several theories as to how it entered the text several centuries after the autograph. One prominent theory is that a manuscript was used for sermons, and a priest may have written notes in the margin from Matthew. A later scribe could then have mistakenly thought an editor to the manuscript was correcting the text with those notes, so they were copied as part of the next generation.
The King James translation came in the early 1600s and was based on 7 known manuscripts from the 11th century and onward. Those texts contained the long ending of Mark. Since that point, nearly 6000 additional manuscripts going as far back as the early 2nd century (and possibly the late 1st century) have been discovered by antiquities traders, archaeologists, and monasteries. The long ending is missing from the oldest, but that was not known in the 1600s. At this point, the inclusion of the long ending is no more than 400 years of tradition.
(Codex Sinaiticus, Mark 16:8 ends column 2)
Long ending problems
Aside from the long ending not appearing in the oldest of the surviving manuscripts, it completely breaks the narrative. The pace of events and the flow of grammar are completely different between Mark 1:1-16:8 and 16:9-20. The Gospel of Mark goes into great detail with many occurrences, yet the long ending abridges a series of events with very little context.
Scripturally, the long ending also runs against the common theme of fear in the rest of the book. "Afraid" appears 8 times, and "fear" appears another 4 times. These all show to some degree the mortality of humanity and how small our piece in the narrative of existence is. Ending Mark at 16:8 leaves with the women being in fear. They had just gone to the tomb of their Messiah, and he was missing! It seems like fear is an appropriate ending.
The original ending
Now, how can it be known that Mark actually ended at 16:8, rather than having a longer ending that has been lost?
First, biblical scholar Daniel B. Wallace points out that Mark would have originally written on a scroll. For scrolls, the ends are preserved the best because they are rolled to the inside, where they are protected from the elements.
Second, though it is common for scribes to miss transcribing a line here or there (which the editors generally catch), but missing 12 entire verses would be very difficult.
Third, when scribes had any doubt about margin notes, they would generally err on the side of addition, as to not redact potentially correct text.
Conclusion
Because of the overwhelming evidence, scholars consider the Gospel of Mark to have originally ended at 16:8, so many modern translations will notate the text as such. Some will even include all 12 verses in footnote form only.
This is very interesting. The Jehovah's Witnesses recently re-translated the New World Translation, in order to use more readily accessible language and to make other various improvements. One of them was the removal of Mark's long ending, which was included as footnote in the previous translation.
@HullBreach
18 Nov 2013 13:35
In reply to Tora.Silver
Your comment about re-translations prompted me to add another paragraph to the "How did it originate?" section.