What About Additions To The End of Mark's Gospel?
It is now recognized among most evangelical Christians that the verses after Mark 16:8 were improperly added. The last page of the folio in Greek was lost. In The Westminster Study Edition of the Holy Bible (Philadelphia: Westminster Press, 1948), the authors explain regarding this passage:
[T]his section is a later addition. The original ending appears to be lost. The best and oldest manuscripts of Mark end with ch. 16:8. [See this link.]
Beginning in the 400s, two different endings were employed after Mark 16:8. One is called the Longer Ending. This appears in the KJV. This includes a verse often used as a proof text that baptism is vital for salvation. We read in Mark 16:16 in the KJV's inclusion of the Longer Ending: "He that believes and is baptized shall be saved; but he that believes not shall be condemned." Catholic authorities believe this section is canonical but admit the "vocabulary and style indicate it was written by someone other than Mark." (Catherine Upchurch, The Four Gospels (Liturgical Press: 2009) at 143.)
The other ending to Mark is known as the Shorter Ending. It exists in many other manuscripts and goes back in its tradition to the 400s as well, having been known to Jerome. [Fn at end or online link.]
Thus, from approximately 400 A.D. to our 20th Century, we have had an addition to Scripture that has gone undetected and treated as canon even though it was certainly written three hundred years after Mark died.
Wikipedia's Fair Analysis
Wikipedia's article Mark 16 provides this synopsis of the evidence:
Textual critics have identified two distinct endings—the "Longer Ending" (vv. 9-20) and the "Shorter Ending," which appear together in six Greek manuscripts, and in dozens of Ethiopic copies. The "Shorter Ending," with slight variations, runs as follows: "But they reported briefly to Peter and those with him all that they had been told. And after this, Jesus himself sent out by means of them, from east to west, the sacred and imperishable proclamation of eternal salvation."
In one Latin manuscript from c. 430, the "Shorter Ending" appears without the "Longer Ending." In this Latin copy (Codex Bobbiensis, "k"), the text of Mark 16 is anomalous: it contains an interpolation between 16:3 and 16:4 which appears to present Christ's ascension occurring at that point; it omits the last part of 16:8, and it contains some strange errors in its presentation of the "Shorter Ending." Other irregularities in Codex Bobbiensis lead to the conclusion that it was produced by a copyist (probably in Egypt) who was unfamiliar with the material he was copying.
Because of patristic evidence from the late 2nd century for the existence of copies of Mark with the "Longer Ending," it is contended by a majority of scholars that the "Longer Ending" must have been written and attached no later than the early 2nd century. Scholars are divided on the question of whether the "Longer Ending" was created deliberately to finish the Gospel of Mark (as contended by James Kelhoffer) or if it began its existence as a freestanding text which was used to "patch" the otherwise abruptly ending text of Mark. Its failure to smoothly pick up the narrative from the scene at the end of 16:8 is a point in favor of the latter option. There is disagreement among scholars as to whether Mark originally stopped writing at 16:8—and if he did so, if it was deliberate or not—or if he continued writing an ending which is now lost. Allusions to a future meeting in Galilee between Jesus and the disciples (in Mark 14:28 and 16:7) seem to suggest that Mark intended to write beyond 16:8.
The writings of the 'fathers' in the era up to the early 300s -- known as the Patristic Era -- appear to sometimes quote from this section of Mark, but it is unclear. The most certain version of the gospels are the complete sets from 325 AD and later-- the Sinaiticus and Vaticanus. On these, Wikipedia reports:
The last twelve verses, 16:9–20, are not present in two 4th-century manuscripts Sinaiticus and Vaticanus, the earliest complete manuscripts of Mark. (Papyrus 45 is the oldest extant manuscript that contains text from Mark, but it has no text from chapter 16 due to extensive damage). Codex Vaticanus has a blank column after ending at 16:8 and placing kata Markon, "according to Mark". There are three other blank columns in Vaticanus, in the Old Testament, but they are each due to incidental factors in the production of the codex—a change to the column-format, a change of scribes, and the conclusion of the Old Testament portion of the text—whereas the blank column between Mark 16:8 and the beginning of Luke is deliberately placed. Although it has been suggested that Codex Vaticanus may be reflecting a Western order of the gospels with Mark as the last book (Matthew, John, Luke, and Mark), the scholars making this suggestion (such as Daniel Wallace) have not explained why any scribe would feel that the normal blank space at the end of a Gospels-codex would be worth perpetuating in a new copy in which the Gospels were arranged in a different order.
Sinaiticus ends with 16:8 and euangelion kata Markon, "the gospel according to Mark," on a page which is part of a replacement-sheet (consisting of four pages) on which the text of Mark 14:54-Luke 1:56 was written by the proof-reader of the manuscript. The text on these four pages was not written by the copyist who wrote the text on the surrounding pages; the pages containing Mark 14:54-Luke 1:56 written by that copyist were removed, and are not extant. (This is unfortunately not mentioned by Metzger; nor is it indicated in the UBS or Nestle-Aland textual apparatus. Nor do they mention Vaticanus' blank column.) On the replacement-pages, the copyist's rate of letters per column varies erratically. At first he wrote normally, but then he used compact lettering until Mark 15:19. At that point, the lettering begins to be written in stretched-out lettering, until the end of Mark 16:8 in column 10. The text of Luke 1:1-56, beginning at the top of column 11, is written in very compact lettering. This indicates that the copyist who made these four replacement-pages in Sinaiticus began by writing the text from Luke (beginning at the top of the 11th column) and then went back to add the text from Mark. After accidentally omitting several lines in 15:47-16:1, he had to stretch his lettering to avoid leaving a blank column between Mark 16:8 and Luke 1:1. Although the copyist, as proofreader, must have seen other blank columns in the codex and considered them unobjectionable, he apparently considered it worthwhile to avoid allowing a blank column to appear between the end of Mark and the beginning of Luke. When this is considered alongside the uniquely emphatic decorative design which follows Mark 16:8 in Sinaiticus, it seems clear that the copyist who made Sinaiticus' replacement-pages was aware of verses 9-20, and desired to prevent the possibility of their inclusion. The copyist who made this replacement-page in Codex Sinaiticus was very probably one of the scribes who helped produce Codex Vaticanus.