Can a true herbivore becomes an omnivore?

We know that true herbivores only eat plant and their digestive system are made and can only cope for this purpose. But I recently found that The Arctic hares according to Wikipedia and other sites could and will occasionally eat meat.

By looking at that example one can say that Arctic hares turn from herbivores to omnivores or they’re actually less-identified omnivores. How could it possible? Can someone here clarify this issue?