The fact is that although the CS is such a valuable manuscript experts note that it contains a very large number of copying errors and, as we have said, shows evidence of many scribes over many years. Michael Marlowe, in his excellent Bible Reseacher Website has commented... "The text of Sinaiticus (written in four columns to the page) contains an unusually high number of readings which have clearly arisen by transcriptional error, most of them by careless omissions." It has some notable omissions such as the loss of the last few verses of Mark's gospel. I have personally examined the copy in the British Library and the end of Mark's gospel has the last 8 verses missing but a space just exactly the right size for them to be inserted! See this image. This seems to indicate that the CS was ever a 'work in progress'.
The CS was found in the St Catherine's Convent at the foot of Mount Sinai in a basket containing scrap papers to be used for kindling the convent's wood fires and rescued by Constans Tischendorf. Its very survival raises some interesting questions. Why did it survive when so many others were 'used until they fell apart'? Was it not used because those who held it had serious doubts about its accuracy? We might ask the same questions about another ancient codex; the Codex Vaticanus. This was found on a shelf in the Vatican and again had not been 'used until it fell apart'.
These two ancient codices were seen as the oldest and therefore the most accurate in the closing years of the 19th century. Consequently the many thousands of other manuscripts were not given their due weight but 'critical scholars' (the experts) have recently begun to give more weight to the testimony of the 'majority' of manuscripts. If you have a copy of the New King James Version you will sometimes find a note which says something like "NU omits..." it is telling you that experts who rely heavily on the Codex Sinaiticus and the Codex Vaticanus do not have these words in their modern translations. This is a complex subject but one that we can discuss later in the New Covenant Discussion Forum.