You know how you confront someone without actually, you know... confronting them? Why, all you have to do is leave a shitty note, that way you communicate your concerns without worrying about having your face punched in.
Actually, that's pretty much a load of bullshit. Passive-aggressive notes are usually written by pussies and laughed at and derided and ultimately ignored by everyone else, however, some of them are so shitty and so pussyish that they beg to be shown around and made fun of.
I guess I should point out that some of these are the result of smartasses too and I respect and love the smartass.