The critical variable here is intent. Jesus expressed the significant difference between knowingly doing something incorrect versus incorrect action through ignorance:
47 And that servant, which knew his lord’s will, and prepared not
himself, neither did according to his will, shall be beaten with many
48 But he that knew not, and did commit things worthy of stripes,
shall be beaten with few stripes. For unto whomsoever much is given,
of him shall be much required: and to whom men have committed much, of
him they will ask the more. (Luke 12:47-48)
The Lord mercifully judges us by our thoughts & intents, rather than just (potentially deceptive) appearances:
7 But the Lord said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the Lord seeth not as man seeth; for man looketh on the outward appearance, but the Lord looketh on the heart. (1 Samuel 16:7)
Let us consider 3 possibilities:
A scribe unintentionally makes a mistake. As already noted by Tony Chan, parablepsis is accidental, not intentional. The same could be said of many other transcription errors. According to the verses cited above, this may indeed have great consequences, but it is not an intentional denigration of God's words, and would not be treated by God as such.
A gloss that is clearly intended as a marginal note. Christians have a very long history of adding marginal notes to their scriptures in order to help them see application, remember certain relevant events/teachings, or make connections. While I recognize that some see this practice as inappropriate (and I can respect that), I do not see any scriptural indication that this is sinful. Would footnotes and commentaries be sinful too?
I see a significant difference between writing a note in the margin of my scriptures for my personal use, versus claiming that the marginal note should be part of the scriptural text.
Intentionally presenting one's own words as the words of another author without authority/permission to do so (e.g. I exclude from this possibility cases where NT writers paraphrased the OT--if they had authority to write what they wrote, God was presumably ok with the paraphrase)--this could be seen as a violation of the 3rd commandment: claiming to speak the words of God, in the name of God, but having no authority to do so. This appears to be the case most applicable to the warning in Revelation 22:18-19 (see also the comparable warning in Deut. 4:2)
It gets complicated
The 3 possibilities above are straightforward, but sometimes it gets messier than this.
It is a common scholarly view that 1 Corinthians 14:34-35 originated as a gloss (perhaps written into the margin of an early copy by a scribe--let's call him scribe 1) that was not part of the letter approved by Paul. A later writer (let's call him scribe 2) incorporated the gloss into the body of the text, and later generations did not realize these 2 verses were interpolations.
Was this an act of wrong-doing? We cannot know for certain the intent of the people involved. Graciously, it is God, who does know their intent, who will be the judge.
Perhaps scribe 1 was trying to insert his views into Paul's letter; perhaps he was making a note about what he believed that he never intended to have published in Paul's name. Perhaps scribe 2 knew better but added a text he liked to the epistle willfully. Perhaps scribe 2 made an honest mistake.
It is my view that textual criticism gives us reason to believe that some manuscript errors were made entirely by accident, and some were intentional. Some may involve a bit of both. God knows the intentions of the heart and will take them into account.
We don't have to wait until Revelation to know that God does not want people to speak in His name without His permission--this is clearly established all the way back in the Torah (e.g. Exodus 20:7)
Sometimes this passage is stretched to mean something it was never intended to mean--which is quite ironic given the content of the passage.
"This book" refers to the book of Revelation, not the Bible. This is evident from:
- Chronology (the Bible as a compilation of 66/73 books didn't exist yet when Revelation was written in the 1st century). Furthermore, Revelation probably wasn't the last book of the Bible to be written
- Deut. 4:2 makes a similar claim--if it signified the end of God's words (as opposed to applying it just to the Torah), all but the first 5 books of the Bible would have to be discarded. The Sadducees & Samaritans might be okay with that, but Jesus was not.
This subject is discussed further in my post here. If we're looking for a statement that God is forever done speaking, the Bible has none to offer. If we're looking for a universal prohibition against speaking in the name of God without authority, the Lord gave it in the 10 commandments.