There are at least 2 instances that I can remember that biblical scholars believe were added on to the original manuscript.
There could be different reasons...there's no way to really know why.
Mark 16:9-20
The external evidence strongly suggests that these verses were not originally part of Mark's Gospel. While the majority of Greek manuscripts contain these verses, the earliest and most reliable do not. A shorter ending also existed, but it is not included in the text. Further, some that include the passage note that it was missing from older Greek manuscripts, while others have scribal marks indicating the passage was considered spurious. The fourth-century church fathers Eusebius and Jerome noted that almost all Greek manuscripts available to them lacked verses
Mar_16:9-20.
The internal evidence from this passage also weighs heavily against Mark's authorship. The transition between verses 8 and 9 is abrupt and awkward. The Greek particle translated "now" that begins verse
Mar_16:9 implies continuity with the preceding narrative. What follows, however, does not continue the story of the women referred to in verse
Mar_16:8, but describes Christ's appearance to Mary Magdalene (cf.
Joh_20:11-18). The masculine participle in verse
Mar_16:9 expects "he" as its antecedent, yet the subject of verse
Mar_16:8 is the women. Although she had just been mentioned three times (
Mar_16:1;
Mar_15:40,
Mar_16:47), verse
Mar_16:9 introduces Mary Magdalene as if for the first time.
Further, if Mark wrote verse
Mar_16:9, it is strange that he would only now note that Jesus had cast seven demons out of her. The angel spoke of Jesus' appearing to His followers in Galilee, yet the appearances described in verses
Mar_16:9-20 are all in the Jerusalem area. Finally, the presence in these verses of a significant number of Greek words used nowhere else in Mark argues that Mark did not write them.
Verses
Mar_16:9-20 represent an early (they were known to the second-century fathers Irenaeus, Tatian, and, possibly, Justin Martyr) attempt (known to the second-century fathers Irenaeus, Tatian, and, possibly, Justin Martyr) to complete Mark's Gospel. While for the most part summarizing truths taught elsewhere in Scripture, verses
Mar_16:9-20 should always be compared with the rest of Scripture, and no doctrines should be formulated based solely on them. Since, in spite of all these considerations of the likely unreliability of this section, it is possible to be wrong on the issue, it is good to consider the meaning of this passage and leave it in the text, just as with Joh. 7:53-8:11.