Not tipping at a restaurant, or even just tipping badly, can be a sure-fire way to go viral on social media. That's because many servers in the U.S. depend on tips for more than half of their earnings. And it's because, as "Shark Tank" investor Kevin O'Leary says, "tipping is an investment."

"You want to establish a relationship with the waitstaff to take care of you," he says.