I've been taught two ways to use a "," in a specific way. What I've learned is that some professors like the "," before the word "and" when there are two different subjects in a sentence, and some do not.
I failed to realize my current professor does not like the "," before "and" in a sentence, therefore on my midterm I lost a considerable amount of points on a portion of the test involving coofftopic placement. I expressed this to her and she said "well now you know how to do it right?"
Of course I just said, "Yep." but I really wanted to say, "No, what I've learned is people who teach English still can't decide whether there should be a coofftopic before "and," and although I've always seen both being accepted you clearly do not."