What would happen if a herbivore ate meat and vise versa?

Apes and monkeys were considered herbivores and later studies showed that they would hunt and eat the flesh of the animals that they caught. Dogs are considered carnivores, but they eat grass etc to calm an upset stomach. Even deer and cattle have been known to partake of animals when the opportunity showed itself. Because animals evolved from common ancestors they will sometimes revert to eating things that we often say aren't in their diets. To often we close out minds to reality and make statements and hold to truths that aren't really truths.

Yes animals can be cannibals. The often are in fact. Dogs , cats and rabbits for example often eat their newborn. We quickly make excuses like, the babies have a deformity or the mother has a nutrition deficit etc. Look at chickens. they will often attack a member of their group and kill and eat it. We quickly call it the pecking order, too often we give them human attributes and don't allow that it is just cannibalism. Read the story of Donner's Pass, we humans ate our own to save our own lives. Isn't that cannibalism? Where do you think the term originally came from? That is what the writers of the Old Testament termed in the first place. This should answer the questions you asked to your satisfaction?

Indigestion. Carnivores do not have the bacteria to help them break down cellulose, and herbivores do not have the strong acids to break down protein. Both of them would end up not digesting their food completely. If you take a look at their feces, you will find a lot of stuff that has not been digested.

They'd starve if forced to only consume the wrong diet.

Like *you'd* starve if you tried to graze.

& NO,though many will consume their own species.

Probably diarhea. lol but foreal.