In a series of three items (e,g. "I have a big, a small and a medium sized dog."), do you include a comma after the second item in the series and before the conjunction (in this example after the word "small")? Is this comma optional?
I personally learned "old school" and use the comma as in "A, B, and C", but even in the dark ages when I was in school it was considered optional and was probably the lesser common choice. Here though is an article discussing some of the reasoning behind continuing to use is, or even selective use of the common in lists: http://www.getitwriteonline.com/archive/052709serialcomma.htm
I would consider the change in meaning to apply only to careful, some might say picky readers, but it can still be real even is subtle.