By pure coincidence I had a similar discussion with my friends yesterday. One claimed that even the most carnivorous animals (sharks, he claimed) eat some vegetables (up to 20%, though that sounds like a lot to me). Wolves apparently like to eat the stomach of their prey first, which in the case of usually herbivorous prey, is likely to contain a herbivorous diet. I read before that deer like to eat the occasional bird.

If true, it seems like there are no true carnivores and herbivores: everybody is omnivorous to some extent. It's just what you get the majority of your diet from.