The longer ending of Mark
The common understanding is that Mark 16:9-20 was not part of the original manuscript.
It is not part of the oldest manuscripts (Vaticanus c. AD 300-325, Sinaiticus c. AD 330-360, and other manuscripts of the Alexandrian text type). It only turns up in the much later Byzantine text type in the 5th century. So most scholars (virtually everyone except KJV-onlyists) reject the authenticity.
"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.”"
Irenaeus of Lyons. (1868–1869). The Writings of Irenæus. (A. Roberts & J. Donaldson, A. Roberts & W. H. Rambaut) (Vol. 1, p. 287). Edinburgh; London; Dublin: T. & T. Clark; Hamilton & Co.; John Robertson & Co.
This is a quote of Mark 16:19, and must have been written prior to AD 202, the year that Irenaeus was martyred.
I'm literally scratching my head right now, wondering whether I should change my mind on the authenticity of the passage…
What do you think: authentic or not?