Wait a second. You’ve been standing in line for 20 minutes, famished and desperate to order one of the viral bagels that keep selling out—or for six hours outside a concert venue, determined to claim a spot at the barricade to see your favorite band.

And then someone strolls up and slips in line ahead of you. Apparently, the rest of you have been standing around for the ambiance.

How much of a sin are line-cutters committing? “You are ripping the fabric of society apart,” says Nick Leighton, an etiquette expert and co-host of the podcast Were You Raised by Wolves? Queue-jumpers have to live with that, he adds. “But worse, we have to live with you.”

Here’s exactly how to handle this maddening situation—including when and how best to speak up.

When to speak up about someone cutting in line

If a stranger jumping ahead of you feels strangely personal, Richard C. Larson, a professor emeritus of data, systems, and society at MIT who has studied queuing, understands why.…