Daniel chapter 4, amazingly, claims to be written by Nebuchadnezzar II of Babylon (642-562 BC). It has Nebuchadnezzar detailing his own humiliation at the hand of Israel's god because of his own pride, and then his restoration by Israel's god - who he calls "the Most High God" (Daniel 4:3, etc.).
His humiliation involves him losing his mind for a period of 7 "times", often interpreted as 7 years (Daniel 4:32-33), after which he is healed.
The following are some problems with this chapter actually having been written by Nebuchadnezzar, or the events described in it having happened:
Nabonidus (615-539 BC) was a king of the Babylonian empire who ruled after Nebuchadnezzar. He was the father of Belshazzar (d. 539 BC). Interestingly, Daniel says that Nebuchadnezzar was the father of Belshazzar (Daniel 5:2, 5:11, 5:18, 5:22).
Nabonidus is the subject of a text found in the Dead Sea Scrolls, called The Prayer of Nabonidus, in which he is made to describe himself going insane for 7 years, and then being healed by "the Most High God" of the Jews:
The Prayer of Nabonidus
1 Words of the prayer, said by Nabonidus, king of Babylonia, [the great] king, [when afflicted]
2 with an ulcer on command of the most high God in Tayma: ["I, Nabonidus,] was afflicted [with an evil ulcer]
3 for seven years, and far from [men] I [was driven, until I prayed to the most high od.] And
4 an exorcist pardoned my sins. He was a Jew from [among the children of the exile of udah, and said:]
5 "Recount this in writing to glorify and exalt the name of [the most high God."Then I rote this:] "When
6 I was afflicted for seven years [by the most high God] with an evil ulcer during my tay at Tayma,
7 I prayed [to] the gods of silver and gold, [bronze and iron,] wood, stone and lime,
8 because [I thought and considered] them gods (source)
Given the extreme similarity with Daniel 4's story about Nebuchadnezzar, it is clear that the two stories about the Babylonian kings are related. However, the question is whether the borrowing is from Nabonidus to Nebuchadnezzar, or vice versa.
Here, the direction of the borrowing can be inferred by recognizing that it would not make sense for someone to apply a story originally about Nebuchadnezzar to Nabonidus, as Nabonidus is a much less famous king, and in the minds of people familiar with both kings, it would sound ridiculous.
However, the opposite is not true. It is very easy to see why someone looking to show Yahweh's dominance over the Babylonian kings would repurpose a legend originally about a marginal king - Nabonidus - which few may have heard, and apply it to the most famous and powerful king which the Babylonian empire ever had, Nebuchadnezzar.
Daniel chapter 4 was not written by Nebuchadnezzar II of Babylon. It was written by a Jew, probably hundreds of years later, who wanted to display Nebuchadnezzar as having been humiliated by his god, Yahweh.
In reality, however, Nebuchadnezzar dominated the Jewish nation while he was the king of the Babylonian empire, and there is no genuine historical evidence that he ever was humbled by the god of the Jews, as Daniel chapter 4 purports.
This has implications for Christianity, as if Daniel chapter 4 is a forgery, then Jesus of Nazareth, who apparently identified himself with a figure in the book of Daniel as one of his central themes (Matthew 24:30, Mark 13:26, Luke 21:27), and appealed to the book of Daniel in preaching (Matthew 24:15, Mark 13:14), was identifying with a figure in a fraudulent book, written at least in part by liars who put words into others' mouths.
This would reveal Jesus of Nazareth to be a man who was deceived by a fraudulent book - a very late addition to the Jewish canon, with many other problems besides the one covered here - and therefore, not a person worth following, as his worldview was warped by fraudulent religious scripture which he mistakenly believed to be authentic.