Did You Mean to Share That Selfie? What to Know About Instagram Instants.

Instagram’s new feature blasts photos to your mutual followers or close friends, immediately.

Instagram’s new feature blasts photos to your mutual followers or close friends, immediately.

Lydia Prior knew that something had gone wrong with her Instagram this week when she got an unexpected notification: a follower had liked her instant.

Ms. Prior, a full-time content creator in Hertfordshire, England, is no stranger to curating Instagram grid posts, stories and even the occasional repost. But what was an instant? She had never heard of it.

She frantically clicked on the notification, thinking, “Oh no, what have I done?”

On the app, she was greeted by a nightmare: a half-dozen unflattering selfies accidentally shared with hundreds of her followers.

“I’ve sent six pictures of my double chin,” she said. “I have no idea what’s going on, and I can’t get rid of them, and everyone kept laughing and being like, ‘Did you do that on purpose?’”

Ms. Prior, 33, was one of millions of Instagram users this week who stumbled upon instants, a new app feature that shares a photo with a person’s mutual followers or close friends — immediately after it’s taken.

Since Instagram rolled out the feature on Wednesday, users have commiserated over its unthinkable side effects, from the threat of an accidental toilet selfie to sharing a snap of the moment they wake up.