* Lee beats himself with a wet noodle *
Yep, you're right, Leightwing. That's an instance of hidden parallel octaves. As Majesty says, in some camps it's excusable; it's less egregious than the straightforward variety. Nevertheless, I invariably try to avoid them, especially in a fairly strict counterpoint exercise like this...except, apparently, when I'm focusing my attention on making an augmentation work. *sigh*
I'm really honoured that you're thinking of using this with your students. Since my goal was to set as good an example as I could, when I get home I'm going to fix it and add it to the errata; but if you'd like to leave it in the version you'd like to show your students as an example, that's fine by me. This is a perfect example of what can happen when you're labouring over one detail without paying attention to all the others.
If anyone finds any other errors, let me know! Thanks!