Emanuel Tov, Textual Criticism of the Hebrew Bible (Minneapolis: Fortress, 2001) 176, 191.
Tov says that there are a variety of texts for the Hebrew Bible that were found at Qumran. There are proto-Masoretic texts, pre-Samaritan texts, Hebrew texts with content similar to what the Septuagint has, and "non-aligned texts which are not exclusively close to any of these groups" (191). Tov states, "Because of the existence of this latter group of texts, it would appear that for every biblical book one could find an almost unlimited number of texts, differing from each other, sometimes in major details."
The "sometimes in major details" part stood out to me. Somewhere in my Tov reading for today (in a passage I can't find right now), Tov says that the differences among the text are usually not that significant. They may involve spelling, or use of a different word that doesn't fundamentally alter the text's meaning. But there are times when the texts differ from each other in "major details." I'm not sure what "major details" Tov has in mind, but perhaps that will become evident as I continue reading.
Bart Ehrman focuses more on the New Testament, and he argues that the different versions and manuscripts sometimes differ in crucial respects. Some texts try to censor out any trace of adoptionism, the idea that Jesus became divine rather than being an eternal member of the Godhead, even before his incarnation. Some present Jesus in Mark 1:41 as angry when a leper approached him, implying (for Ehrman) that he was mean and impatient, the type who didn't want to be bothered (though, in my humble opinion, maybe Jesus was angry at the existence of disease). Other texts for Mark 1:41 portray a more compassionate Jesus. If my memory serves me correctly, Ehrman thinks that the latter was a correction of the former. But, take heart! Ehrman doesn't always argue that nice Jesus was a scribal correction of mean Jesus, for he thinks that the reverse can occur as well. For example, he contends that many manuscripts leave out Luke 23:34 ("Father, forgive them, for they know not what they do") out of a dislike for Jews, implying that Luke 23:34 (forgiving Jesus) was the earlier reading.
As I shared in my posts, Updating and Text Criticism in Antiquity and A Humble God Who Stands, Our Stance Towards the World, there were rabbis who asserted (either correctly or incorrectly) that certain people revised the Torah. One rabbinic text narrates that the translators who produced the Septuagint tried under divine inspiration to eliminate apparent errors or rewrite passages that could be construed to support heresy (e.g., the notion that there are two powers in heaven, which, for many rabbis, contradicted Jewish monotheism). Another states that scribes changed a passage in Genesis 18 out of respect for God, but the author of that particular rabbinic statement prefers the earlier version because it makes God look humble. According to certain rabbis, changes have been made to the biblical text out of a concern for crucial issues, such as orthodoxy and the honor of God. Some rabbis support the changes, even going so far as to assert that they occur under divine inspiration; others prefer the earlier text.
Tov discusses a possible alteration of the text over a theological issue, but he isn't particularly dogmatic about this. There are two versions of I Samuel 1:23. The context of the passage is that Hannah has just given birth to Samuel, whom she promised to give to God in her prayer for a son. Hannah does not want to take baby Samuel to the sanctuary in his first year, for he's not been weaned yet. Her husband Elkanah then tells her to do what she deems to be good, and to go ahead and wait for the child to be weaned. According to the Masoretic Text, Elkanah then says, "May the LORD fulfill His word." In Qumran text 4QSamA, he says, "[May the LO]RD [fulfill] that which comes out of your mouth."
What's going on here? Why do these two different versions exist? Are both of the texts "original texts," with neither being derived from the other? Or do two texts exist because one of them was a new version that sought to "correct" the other? Did the Qumran text aim to correct the MT's "His word" to "that which comes out of your mouth" because the story of Hannah never mentions the word of the LORD, but only Hannah's vow? Plus, the topic of vv 22-23 was Hannah's vow to take her child to the sanctuary. Or was the MT's "His word" a pious attempt to correct the Qumran text, since "the mentioning of the 'word' of God reflects more reverence towards God than the vow of a mere mortal, Hannah" (176). Although Tov is agnostic about which possibility is correct, he acknowledges that some variations in texts can have a theological underpinning, even though the issue here isn't as crucial as whether or not there are two powers in heaven!
Then, historical-critics would argue that the Bible is full of new ideas rejecting or modifying older concepts, and yet both are in the Bible. But that's a big issue by itself!