The update, which was quickly reversed, appeared to affect iPhone users but not those with Androids or those using the app on desktop.

In a series of tweets, Adam Mosseri, the head of Instagram, explained that the new feature was a “test” that was released widely by mistake.

“Just a test that went to a few orders of magnitude more people than intended... sorry about that,” he wrote in one tweet.

The company affirmed Mr. Mosseri’s explanation and apologized.

“Due to a bug, some users saw a change to the way their feed appears today,” Instagram said. “We quickly fixed the issue and feed is back to normal. We apologize for any confusion.”