"One of the problems with ancient Greek texts (which would include all the earliest Christian writings, including those of the New Testament) is that when they were copied, no marks of punctuation were used, no distinction made between lowercase and uppercase letters, and, even more bizarre to modern readers, no spaces used to separate words." (pg. 48)
Think of the complexity of some of the New Testament, like Paul's letters (I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good.). Even with modern rules of grammar imposed, this passage is pretty ridiculous. When those rules are done away with, it's easier to misread what is on the page. And even if your eyes and brain play no tricks, even if every letter is read in its correct place, comprehension and translation errors can still happen. Ehrman provides an example of what can happen with the following word: "godisnowhere", which can be read as "God is now here" or "god is nowhere".
"Far and away the most changes are the result of mistakes, pure and simple -- slips of the pen, accidental omissions, inadvertent additions, misspelled words, blunders of one sort or another. Scribes could be incompetent: it is important to recall that most of the copyists in the early centuries were not trained to do this kind of work but were simply the literate members of their congregations who were (more or less) able and willing." (pg. 55)
Mistakes happen, but once they do, it can be hard to figure out where they are and how to correct them. And now, centuries later, how can we ever expect to know for certain which manuscript is the most correct version?
"What survives today, then, is not the original copy of the letter, nor one of the first copies that Paul himself had made, nor any of the copies that were produced in any of the towns of Galatia to which the letter was sent, nor any of the copies of those copies. The first reasonably complete copy we have of Galatians (this manuscript is fragmentary; i.e., it has a number of missing parts) is a papyrus called P46 (since it was the 46th New Testament papyrus to be catalogued), which dates to about 200 C.E. That's approximately 150 years after Paul wrote the letter. It had been in circulation, being copied sometimes correctly and sometimes incorrectly, for fifteen decades before any copy was made that has survived down to the present day." (pg. 60)
"Original" scripts for the Bible are essentially nonexistent, and even the earliest ones that we have are so far from the original that there is no way to determine how accurate the copies are.