Spy photos

It's not an incorrect comma as such. When you list a number of 'things', you separate each one with a comma, except the penultimate one, which gets attached to the last one with "and". 
e.g. For her birthday, she wants winter gloves, a drysuit, a warm jacket and a Contender.

Now, if the list is too long or if the items listed are "complex" (like a composed noun), you might want to mark the pause after the penultimate (one before last) with a comma.
e.g. For her birthday, she wants winter gloves, a drysuit, a Contender with a Wavelength carbon tiller extension, and a warm jacket.

The comma placed between 'extension' and 'and' is an Oxford comma. It makes the sentence easier to understand but, in my view, shouldn't be overused. American journalists tend to use the Oxford comma quite freely, which is another reason why we shouldn't overdo it.

I'm off with my camera to find an Oxford comma in its natural habitat and bring back a photo for this forum. These creatures are rather shy, so I might be gone for a while.

If anyone knows why it's Oxford, rather than Cambridge or errr... Milton Keynes , please let us know, thx.   

 Ohh there's the bell! End of the lesson for this week. There's going to be a quick test next time, so don't forget your revision, thank you