Is it okay for cats and dogs to eat each other's food occasionally?

Quick answer

Stealing a bite here and there isn't a problem, but long-term feeding is never okay. Cats in particular need food containing taurine, an essential amino acid they can't synthesize on their own. Continued feeding of dog food can cause taurine deficiency in cats, leading to blindness or life-threatening heart disease (dilated cardiomyopathy).