Instagram surprised users on Thursday with a sudden change — a feed that scrolled from left to right, rather than its traditional vertical format.

The test led to immediate backlash on social media, but users had a quick resolution. The head of Instagram announced that the change had been meant as a small test, but a mistake caused it to roll out to a large number of users.

"That was supposed to be a very small test that went broad by accident. Should be fixed now. If you're still seeing it simply restart the app. Happy holidays!" tweeted Adam Mosseri, head of Instagram, shortly after the test caused a wave of backlash on Twitter.

According to screenshots shared by Instagram users on Twitter, the update is similar to how people view Instagram stories.

The update triggered an immediate reaction on social media, with users flooding Twitter with their grievances, causing the hashtag "instagramfail" to trend.

Mosseri further explained that the app, which has more than 1 billion monthly users, is constantly testing new experiences but that Thursday's change went to "a few orders of magnitude more people than intended ... "

Thursday’s surprise update — which mirrored uproar Snapchat received earlier this year when it redesigned how its users see stories on the app — wasn’t the only recent change for Instagram.

In September, CEO Kevin Systrom and Chief Technical Officer Mike Krieger announced in a statement that they were leaving “to explore (their) curiosity and creativity again.”

Mosseri, who had previously been head of Facebook's "News Feed," took over Instagram in October.