Dog eating cat food

My 7 year old Pekingese loves to eat cat food. He still eats his Science Diet dog food as well, but is it damaging to his health if he eats a little cat food once in a while? Also, I read someone on here say they knew cat food was bad for dogs, but why is that? I mean, how does it harm them? If it's really horrible I won't allow him to eat it anymore, but he just enjoys cleaning up what the cats don't eat at dinner.
Also, if anyone knows any good sources of info on this topic, please point me in the right direction. I haven't had much luck just searching online.
Thank you for any respones!

[Aleda M Cheng, D.V.M., C.V.A]

As long as he doesn't get any vomiting or diarrhea, and is at his ideal weight it is alright.  The increased protein and fat in cat food can sometime cause GI issues and pancreatitis in dogs.  Most commercial cat food is not even ideal for cats since a lot of cat foods contain grain is a variety of form such as meal,  and glutin.  Cat's do not have any need in their diets for grains of any kinds.