BENGALURU—Travel booking platform Cleartrip took out full-page newspaper ads this month to apologize to other companies in advance for the employees its sale was about to take from them.

The ads ran Oct 10 in The Economic Times and The Gurgaon Times. They apologize to brands for the empty offices expected during the company's Big Billion Days sale, which offers fares so low that workers are expected to suddenly come down with something.

"We believe this is the first sale in history to generate its own sick notes," said Priya Venkat, an HR consultant. She said she had already received 2,000 doctor's notes for a condition listed only as "Goa."

Employers responded with alarm. Operations manager Arjun Mehra said his team of 40 had shrunk to three people and a ficus. "Usually you hear about a staff shortage after it happens," he said. "This is the first time someone has apologized for one in advance and then sold it to you at 40% off."

Analysts said the campaign is a breakthrough in apology marketing. An apology has traditionally been how a company says it is sorry. It is now how a company announces it plans to do something worth being sorry about.

At press time, Cleartrip was reportedly drafting a second ad apologizing for the first one working.