Pete, in the early 19th century, Daniel Webster unilaterally decided that America being its own new country should have its own new language. So he wrote grammar books with spellings he invented which eliminated letters he considered to be superfluous.
These textbooks were used by Canadians. There was a wave of Scottish immigration to Canada, and the Scots were horrified to see these textbooks, saying that the Canadians were teaching their children error. They insisted that proper British spelling and grammar be taught to Canadian children, and it has been ever since.
I don't consider American spelling to be error because it is the result of a deliberate decision to be different.
The error I am referring to is typically the practice of those who don't know any better, and often (I suspect) the practice of those who don't read.