Instagram has quietly started testing a horizontal feed for some users, a huge sideways shift from the vertically scrolling user experience that’s been the norm since the app launched. The company had previously been testing the feature back in October in the Explore section of the app, but it seems that it’s rolling it out more widely to users’ main feeds today.

The new feed basically turns all posts into a single, giant Instagram story, complete with tapping to advance and a scrolling bar at the top to show you how far you’ve progressed. It’s certainly a jarring change for Instagram’s community, which has grown accustomed to the old feed.

With the horizontal feed, each photo (and each ad) is given the spotlight at any given time — you can only view a single post, and the comments are much more prominent now, appearing with a flick upward.

It’s also much harder to quickly fly through your feed with the new layout: you can only advance a single post at a time. Another side effect of the new layout is that stories are easier to access, with the bar now always available by swiping down, instead of requiring that you scroll all the way to the top of your feed.

The new horizontal layout seems only to affect the main feed — tap into a user’s individual profile, and things will appear as they always have in the usual, vertically scrolling style.

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Instagram head Adam Mosseri commented on Twitter that the horizontal feed was meant to be “a small test” that “went broader than we anticipated.” But a Facebook spokesperson contradicted that in a statement to The Verge: “Due to a bug, some users saw a change to the way their feed appears today. We quickly fixed the issue and feed is back to normal. We apologize for any confusion.”

Only a few minutes after the horizontal feed went live, the old-fashioned vertical feed seems to have reappeared for most of the users that had been seeing the horizontally scrolling test. But even if today’s update was an accident, it seems that Instagram is taking the horizontal feed seriously — and it may not be long before it rolls out to devices for good.

Update December 27th, 11:35AM ET: Instagram seems to have reverted users back to the vertical feed. Also added additional details, comment from Instagram founder, and official statement from Instagram.