The process you described is correct. However, the problem here is that the enzymes aren't perfect and coding errors are quite common (they get fixed, sometime

The process you described is correct. However, the problem here is that the enzymes aren't perfect and coding errors are quite common (they get fixed, sometimes). A big problem is that some chemicals can look like a nucleotide (A or T or C or G) and after insertion it can "decay" into a different one, hence causing an error. During replication it is possible for a DNA fragment to be cleaved (once again, it is often repaired, but thats how the Y chromosome came to be AFAIK) however sometimes enzymes "mess up" and reattach them at wrong positions. At other times base-pairs are deleted or inserted shifting the whole strand. There is a lot of things that could go wrong.