Like the Old Testament (OT) text, there exists various types of corruption in the New Testament (NT) text as well. The scholarly book named 'The Text of the NT, its Transmission, Corruption, and Restoration' deals exclusively with this theme. The heading of Chapter VII of the book is "The Causes of Error in the Transmission of the Text of the NT". It would be very pertinent to undertake a study of its themes. The learned writer has divided it into two sub-headings:
(I) Unintentional Changes
(II) Intentional Changes
In the following paragraphs both these types of changes shall be described under various sections:
I. Unintentional Changes
01. Errors Arising from Faulty Eyesight
(a) The scribe who was afflicted with astigmatism found it difficult to distinguish between Greek letters which resemble one another, particularly when previous copyists had not written with care. Thus in the uncial script the sigma, the epsilon, the theta, and the omicron were sometimes confused, etc.
(b) When two lines in the exemplar from which a scribe was making a copy happened to end with the same word or words, or even sometimes with the same syllable, his eye might wander from the first to the second, accidentally omitting the whole passage lying between them. Thus is to be explained the curious reading at John xvii, 15 in Codex Vaticanus, which lacks the words which are enclosed in square brackets: 'I do not pray that thou shouldst take them from the [world but that thou shouldst keep them from the] evil one.' (...). Many other examples of omission, called haplography, occur in a wide variety of MSS. (…). Sometimes the eye of the scribe picked up the same word or group of words a second time and as a result copied twice what should have appeared only once (this kind of error is called dittography).
02. Errors Arising from Faulty Hearing
When scribes made copies from dictation, or even when a solitary scribe in his own cell pronounced to himself the words which he was transcribing, confusion would sometimes arise over words having the same pronunciation as others, but differing in spelling (as the English words 'there' and 'their' or 'grate' and 'great').
03. Errors of the Mind
The category of errors of the mind includes those variations which seem to have arisen while the copyist was holding a clause or sequence of letters in his (somewhat treacherous) memory between the glance at the MS to be copied and the writing down of what he saw there. In this way, one must account for the origin of a multitude of changes involving the substitution of synonyms, variation in the order of words, and the transposition of letters. (….).
04. Errors of Judgment
Words and notes standing in the margin of the older copy were occasionally incorporated into the text of the new MS. Since the margin was used for glosses (that is, synonyms of hard words in the text) as well as corrections, it must have often been most perplexing to a scribe to decide what to do with a marginal note. It was easiest to solve his doubt by putting the note into the text which he was copying. Thus it is probable that what was originally a marginal comment explaining the moving of the water in the pool at Bethesda (John v.7), became incorporated into the text of John v. 3b-4 (see the KJV for the addition). Again, it is altogether likely that the clause in later MSS at Rom. viii. 1, 'who walk not according to the flesh but according to the spirit', was originally an explanatory note (perhaps derived from vs. 4) defining 'those who are in Christ Jesus'. (…). Other errors originated, not because of the exercise of faulty judgment, but from lack of judgment altogether. 
II. Intentional Changes
Odd though it may seem, scribes who 'thought' were more dangerous than those who wished merely to be faithful in copying what lay before them. Many of the alterations which may be classified as intentional were no doubt introduced in good faith by copyists who believed that they were correcting an error or infelicity of language which had previously crept into the sacred text and needed to be rectified. A later scribe would even re-introduce an erroneous reading that had been previously corrected.
01. Changes Involving Spelling and Grammar
The Book of Revelation, with its frequent Semitisms and solecism, afforded many temptations to style-conscious scribes. [The writer has given here some concrete examples of the Greek language to elaborate the theme].
02. Harmonistic Corrections
Some harmonistic alterations originated unintentionally; others were made quite deliberately. Since monks usually knew by heart extensive portions of the Scriptures, the temptation to harmonize discordant parallels or quotations was strong in proportion to the degree of the copyist's familiarity with other parts of the Bible. The words which belong in John xix. 20, 'It was written in Hebrew, in Latin, and in Greek', have been introduced into the text of many MSS at Luke xxiii. 38. (…). Frequently, OT quotations are enlarged from the OT context, or are made to conform more closely to the Septuagint wording. For example, the clause in the King James version at Matt. xv. 8, '[this people] draweth nigh unto me with their mouth' – a clause which is not found in the earlier MSS of Matthew – was introduced into later MSS by conscientious scribes who compared the quotation with the fuller form in the Septuagint of Isa. xxix. 13. [There are other examples in it as well to elaborate the point].
03. Addition of Natural Complements and Similar Adjuncts
The work of copyist in amplifying and rounding off of phrases is apparent in many passages. Many scribes supposed that something was lacking in the statement in Matt. ix. 13, 'For I came not to call the righteous, but sinners', and added the words 'unto repentance' (from Luke v. 32). So, too, many a copyist found it hard to let 'the chief priests' pass without adding 'the scribes' (e.g. Matt. xxvi. 3), or 'scribes' without 'Pharisees' (e.g. Matt. xxvii. 41); or to copy out the phrase 'your Father who sees you in secret will reward you' (Matt. vi. 4, 6), without adding the word 'openly'. (…). A good example of a growing text is found in Gal. vi. 17, where the earliest form of the text is that preserved in 'I bear on my body the marks of Jesus'. Pious scribes could not resist the temptation to embroider the simple and unadorned with various additions.
04. Clearing up Historical and Geographical Difficulties
In earlier MSS of Mark i.2 the composite quotation from Malachi (iii.1) and from Isaiah (xl. 3) is introduced by the formula, 'As it is written in Isaiah the prophet'. Later scribes sensing that this involves a difficulty, replaced [the writer has given here the Greek words] with the general statement [the writer has given here the actual Greek words]. Since the quotation which Matthew (xxvii.9) attributes to the Prophet Jeremiah actually comes from Zechariah (xi. 12f.), it is not surprising that some scribes sought to mend the error, either by substituting the correct name or by omitting the name altogether. A few scribes attempted to harmonize the Johannine account of the chronology of passion with that in Mark by changing 'sixth hour' of John xix. 14 to 'third hour' (which appears in Mark xv. 25). At John i. 28 Origen altered …to … in order to remove what he regarded as a geographical difficulty, and this reading is extant today in MSS and many others, including those which lie behind the KJV. The statement in Mark viii. 31, that 'the Son of man must suffer many things … and be killed and after three days rise again', seems to involve a chronological difficulty, and some copyists changed the phrase to the more familiar expression, 'on the third day'.
The author of the Epistle to the Hebrews places the golden altar of incense in the Holy of Holies (Heb. ix. 4), which is contrary to the OT description of the Tabernacle (Exod. xxx. 1-6). The scribe of codex Vaticanus and the translator of the Ethiopic version correct the account by transferring the words to ix. 2, where the furniture of the Holy Place is itemized.
05. Conflation of Reading
What would a conscientious scribe do when he found that the same passage was given differently in two or more MSS which he had before him? Rather than making a choice between them and copying only one of the two variant readings (with the attendant possibility of omitting the genuine reading), most scribes incorporated both readings in the new copy which they were transcribing. This produced what is called a conflation of readings, and is characteristic of the later, Byzantine type of text. For example, in some early MSS the Gospel according to Luke closes with the statement that the disciples 'were continually in the temple blessing God', while others read 'were continually in the temple praising God'. Rather than discriminate between the two, later scribes decided that it was safest to put the two together, and so they invented the reading 'were continually in the temple praising and blessing God'.
In the early MSS at Mark xiii. 11 Jesus counsels his followers not to be 'anxious beforehand' (…). Other MSS of Mark read 'do not practice beforehand' (…), which is the expression used also in the Lucan parallel (xxi. 14). Rather than choose between these two verbs, a good many copyists of Mark gave their readers the benefit of both. In Acts xx. 28 the two earlier readings, 'church of God' and 'church of the Lord', are conflated in later MSS, producing 'the church of the Lord and God'. 
06. Alterations Made because of Doctrinal Considerations
The number of deliberate alterations made in the interest of doctrine is difficult to assess. Irenaeus, Clement of Alexandria, Tertullian, Eusebius, and many other Church Fathers accused the heretics of corrupting the Scriptures in order to have support for their special views. In the mid-second century, Marcion expunged his copies of the Gospel according to Luke of all references to the Jewish background of Jesus. Tatian's Harmony of the Gospels contains several textual alterations which lent support to ascetic or encratite views.
Even within the pale of the Church one party often accused another of altering the text of the Scriptures. Ambrosiaster, the fourth-century Roman commentator on the Pauline Epistles, believed that where the Greek MSS differed on any important point from the Latin MSS, which he was accustomed to use, the Greeks 'with their presumptuous frivolity' had smuggled in the corrupt reading. In revising the Old Latin text of the Gospels, St. Jerome was apprehensive lest he be censured for making even slight alterations in the interest of accuracy – a fear that proved to be well founded!
The MSS of the NT preserve traces of two kinds of dogmatic alterations: those which involve the elimination or alteration of what was regarded as doctrinally unacceptable or inconvenient, and those which introduce into the Scriptures 'proof' for a favorite theological tenet or practice.
In transcribing the prologue to the Third Gospel, the scribes of several Old Latin MSS as well as the Gothic version obviously thought that the Evangelist should have referred to divine approval of his decision to compose a Gospel, and so to Luke's statements (i.3), 'It seemed good to me . . . to write an orderly account . . .', they added after 'me' the words 'and to the Holy Spirit'. The addition imitates the text of Acts xv. 28, which reads, 'For it has seemed good to the Holy Spirit and to us . . .'.
The inconsistency between Jesus' declaration in John vii. 8, 'I am not going up to the feast, for my time has not yet fully come', and the statement two verses later: 'But after his brothers had gone up to the feast, then he also went up, not publicly but in private' (a discrepancy which Porphyry seized upon to accuse Jesus of 'inconstantia ac mutatio'), led some scribes to change ... ('I am not yet going up ...'). Jesus' statement: 'But of that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only' (Matt. xxiv. 36 and Mark xiii. 32), was unacceptable to scribes who could not reconcile Jesus' ignorance with his divinity, and who saved the situation by simply omitting the phrase (….).
In Luke ii there are several references to Joseph and Mary which, in the ordinary text, doubtless appeared to some persons in the early Church to require rephrasing in order to safeguard the virgin birth of Jesus. In ii. 41 and 43 instead of the words 'his parents' some MSS read 'Joseph and Mary'. In ii. 33 and 48 certain witnesses alter the reference to Jesus' father either by substituting the name Joseph (as in vs. 33) or by omitting it altogether (as in vs. 48). 
It can thus be safely concluded that the text of the NT had to suffer many types of setbacks due to a number of reasons, some of which are detailed above. As such, all possible analytical and critical measures should be adopted to ascertain the validity and intent of its text. At the same time, with all its shortcomings, the NT has preserved a lot of theological, historical, and prophetic substance in it and is not to be discarded outright.