Suppose, though, that the scribe got all the words 100 percent correct. If multiple copies of the letter went out, can we be sure that all the copies were also 100 percent correct? It is possible, at least, that even if they were all copied in Paul's presence, a word or two here or there got changed in one or the other of the copies. If so, what if only one of the copies served as the copy from which all subsequent copies were made — then in the first century, into the second century and the third century, and so on? In that case, the oldest copy that provided the basis for all subsequent copies of the letter was not exactly what Paul wrote, or wanted to write.
Once the copy is in circulation — that is, once it arrives at its destination in one of the towns of Galatia — it, of course, gets copied, and mistakes get made. Sometimes scribes might intentionally change the text; sometimes accidents happen. These mistake-ridden copies get copied; and the mistake-ridden copies of the copies get copied; and so on, down the line. Somewhere in the midst of all this, the original copy (or each of the original copies) ends up getting lost, or worn out, or destroyed. At some point, it is no longer possible to compare a copy with the original to make sure it is "correct," even if someone has the bright idea of doing so.
Suppose that after the original manuscript of a text was produced, two copies were made of it, which we may call A and B. These two copies, of course, will differ from each other in some ways — possibly major and probably minor. Now suppose that A was copied by one other scribe, but B was copied by fifty scribes. Then the original manuscript, along with copies A and B, were lost, so that all that remains in the textual tradition are the fifty-one second-generation copies, one made from A and fifty made from B. If a reading found in the fifty manuscripts (from B) differs from a reading found in the one (from A), is the former necessarily more likely to be the original reading? No, not at all — even though by counting noses, it is found in fifty times as many witnesses. In fact, the ultimate difference in support for that reading is not fifty manuscripts to one. It is a difference of one to one (A against B). The mere question of numbers of manuscripts supporting one reading over another, therefore, is not particularly germane to the question of which reading in our surviving manuscripts represents the original (or oldest) form of the text. B. Ehrman, Misquoting Jesus (HarperCollins, 2005), 59, 128-129.
This is one of Ehrman's stock objections to the authenticity of the NT text, as we have it today. He repeats variations of this objection in his debates.
The argument appears to undercut the common apologetic appeal to the number of Greek MSS and even the antiquity of some Greek MSS. Although we have lots of MSS, if these derive from the same copy, that really counts as one rather than many. Likewise, although some of our MSS are very early, if they derive from the same defective parent copy, their antiquity doesn't make them reliable. I've discussed this before, but I'd like to say a bit more about the issue.
i) We've all seen letter boards. These are signs with movable letters. You have a box with magnetic letters of the alphabet. That way you can change the message on the sign when you have a new product or service to advertise.
We've all seen signs in which one or more of the letters dropped off. Sometimes the effect is comical. It changes the meaning of the message. However, it's usually easy to figure out the original message. If you know the language (e.g. English, Spanish), if you know the context, you can mentally reconstruct the intended message. This is something we all do. You don't need to have access to the original as a basis of comparison. Ehrman is overlooking really obvious counterexamples to his facile objection.
ii) Another problem with his objection is that we have four Gospels, not merely one. So he'd have to postulate that the chain of transmission was garbled, not just once, but independently for all four gospels.
iii) Ehrman has a "heads I win, tails you lose" approach to the Gospels. If they're different from each other, that's a contradiction! But if they agree, that's not independent multiple attestation. Rather, that just means Christians were telling each other the same stories, which eventually got written down. He's rigged it so that nothing can ever count as evidence for the historical Jesus.