The idea that the earliest copiers of the OT, officially called the Scribes, were vastly accurate in their work is a fallacy perpetuated by evangelical apologists.
As we can see in the construction of the first five books of the Old Testament, this compilation of books had been edited and changed from it's inception by a redactor in the 5th century BCE. Jeremiah who was possibly one of the authors of the Pentateuch complained in Jer 8:8 that the scribes who's role it was to copy these texts had a "lying pen" and were corrupt. Even Jesus is reported to have likened the Scribes to the Pharisees and rebuked them publicly for being hypocrites and preventing men from entering heaven. What we can see from earlier textual evidence is that priests from Judah and Israel were fiercely opposed to each other and saw each other as heretical. This would have led them to alter texts to suit their own religious or political purposes. So we can clearly see that their reputation was already in jeopardy but what about the textual evidence?
It is said that when the Babylonians invaded Israel in 537 BCE, they destroyed many or possibly all of the existing manuscripts of the OT. When the Roman followed suit in 70 CE, we once again see many of these being manuscripts destroyed.
The earliest complete Hebrew text of the first five books of the OT that we have today is the Samaritan Pentateuch. This dates to as late as the 11th century CE. It's worth noting that this Hebrew text differs from the Orthodox Hebrew text in 6000 places. It differs even more the Septuagint which is a earlier Greek translation of the OT.
The Septuagint was a Greek version of the OT, which was copied into Greek in Alexandria in the 3rd century BCE. This task was undertaken by Hellenized Jews who's audience were no longer familiar with the classical Hebrew used in the OT. They had the challenge of modernizing the OT Hebrew and reinterpreting it for their Greek speaking Jewish audience.
The earliest copy we have of this document is in the Codex Sinaiticus and Codex Vaticanus manuscripts which dates to the 4th century CE. These copies themselves are based on an earlier version developed by Origen (186-254 CE) who had reviewed the various Greek OT manuscripts at his disposal and authored his own version of the bible called the Hexapla. While Origen was reviewing the Septuagint he noticed that it's genealogy of David differed from Luke in two places. Favouring Luke, Origen changed the text in the Septuagint to reflect a more Christian friendly version. Thus we can see variants in the versions of the Septuagint in the 2nd century CE, however we have no idea how true these are to the original Greek versions which were translated 500 years earlier in the 3rd century BCE. We also do not even know what the 3rd century Hebrew texts looked like, which were used by these scribes in translating these texts. And let's not forget Jeremiah's warning regarding the "lying pen" of the scribes and that there were already problems in regards to errors in the Hebrew manuscripts in the 6th century BCE.
It is important to note that the Septuagint was the only version of the Old Testament used by the New Testament authors as well as the early church. Matthew, Mark, Luke and Paul all quote from the error prone Greek Septuagint and not the Hebrew OT versions, which is interesting in and of itself.
The versions of the Septuagint that we have today also differ vastly from the Orthodox Hebrew OT as well as the Masoretic Text. One example is that the book of Jeremiah in the Septuagint is one eight shorter than it is in the Masoretic text.
The Masoretic text was developed following the destruction of the Temple in Jerusalem in 70 CE. Most of the OT manuscripts had been destroyed during this conflict and Hebrew scribes worked over the next two centuries in developing an authoritative Hebrew version of the Torah. This version is today called the Masoretic text or the Orthodox Hebrew text.
Dead Sea scrolls were discovered in 1947 and these contained very early pre-Masoretic fragments of the Old Testament. Some of the texts found are very similar to the Septuagint and others resemble the Masoretic text. This shows that this was simply one of the variants of OT texts in existence.
So as you can see we today have vastly different versions of the Old Testament which are based on manuscripts of varying textual integrity. The Samaritan Pentateuch, Septuagint, Masoretic and the Dead Sea scrolls all differ from one another. Sometimes these differences are in their spelling and on other occasions these differences are much larger. This led to various conflicts between Catholics and Protestants, Christian and Hebrew, conservative scholars and liberal scholars in regards to which text is the most authoritative. One thing is certain and that is that none of these versions are 100% similar to the "original" Hebrew versions, whatever they might have looked like and this notion of there being no differences in the OT copies is a myth itself.