Psalm 151

Psalm 151 is a short psalm that is controversial because it is in some versions of the Bible but not others. In particular, it is in several very old Greek translations of the Bible but is not in the Hebrew Bible. There has been a lot of debate over whether or not the psalm should be considered part of the Bible.
Psalm 151 and the Dead Sea Scrolls.
The psalm has been known about for a long time and the Greek version was used for study for hundreds of years. When the Dead Sea Scrolls (which are in Hebrew) were found and translated, part of Psalm 151 was in them. This surprised the researchers very much, but not as much as what they would find later on.
After studying the scrolls, they noticed that the Greek translator had actually made a mistake. Two different pieces of two different psalms were treated like they were one big psalm when they were not. There is a big change in the Greek version after the fifth line (where the second psalm fragment would begin) and is a lot more like a story than most of the other psalms, which are more like poems.
Content.
The Greek version of the psalm says that it was written by King David after his battle with Goliath. It talks about how, even though David was the least of all his brothers, God chose him to be king. It then talks about how David cut off Goliath's head with Goliath's own sword after killing him with his slingshot.
Status today.
The Eastern Orthodox Church accepts Psalm 151 as part of the Bible, but Roman Catholics, most Protestants, and most Jews do not. However, it can be found in some Catholic Bibles and in certain other Bible translations like the New Revised Standard Version.