If your puppy occasionally sneaks some of Kitty's food, that shouldn't be a problem. Feeding him cat food on a regular basis in lieu of proper puppy food is another story. Growing canines require food designed for their nutritional needs to develop properly, not that of another species.
The Right Diet
Cat food contain higher levels of protein, fat and calories than dog and puppy foods. That might make feline foods even more attractive to puppies -- it's sort of like asking kids whether they'd rather eat ice cream or vegetables. While puppy foods do contain more protein than adult dog foods, cat food doesn't contain the minerals and vitamins growing dogs need. Feeding your puppy cat food can quickly make her overweight, which isn't healthy.
If you find a stray puppy late at night, you can give him some cat food to tide him over if that's all you have. If you run out of food for your own puppy, you can feed him cat food in a pinch. In either case, purchase some puppy food as soon as possible. It's possible a puppy might suffer from diarrhea after eating cat food. Minor loose bowels usually clear up once a puppy receives the right food, but severe diarrhea requires a trip to the vet for treatment.