For a few versions of Word, Fast Save defaulted On in the options. This was a problem for several reasons. First, by adding changes to the end of the file, it made documents inordinately larger than necessary whilst not saving a great deal of time saving them. Secondly, it made the document format more fragile than necessary. Thirdly, it meant deleted text was still present in the file! This was fuel for a number of embarrassing episodes in the last decade. And lastly, the extra complexity to the format meant it took anti-virus vendors longer than they liked to be able to clean documents of macro virii..
In short, Fast Save was one of Microsoft's silent fiascos. Fortunately, more recent versions seem to default it to Off, but the option is still offered.
Wade.