Imagine this: You’re messaging a friend on Facebook, one you haven’t seen in a while, trying to make plans to catch up. Maybe you’re walking while you type, or are just a little distracted, and you go to press send on another message.

Suddenly, though, and seemingly out of nowhere, a video ad starts playing on your screen; maybe you accidentally click it, immediately and inadvertently sending you straight to a download page or even into another app. In a matter of seconds, you’re no longer letting Karen know, yeah, you’re free for lunch at one, but unexpectedly downloading the twelfth iteration of an overwrought Words With Friends.

Sadly this fever dream, this nightmare, is not just that. It’s your brand new user experience on Facebook’s messaging platform, Messenger.

From his “beachside cabana” at the Cannes Lions advertising festival on Tuesday, head of ad business for Messenger Stefanos Loukakos announced that Facebook had already begun rolling out auto-playing video advertisements within Messenger at the start of this week. Unlike the regular banner ads you get on your Facebook sidebar or the static advertisements you’ll have seen on Messenger since the start of last year, the new ads will be autoplaying video pop-ups appearing smack-dab in the middle of all your conversations.

The roll-out doesn’t just mark another invasive ad strategy from a social media platform, but the first major change, arguably ever, to Messenger’s user experience.

For a lot of people, this will not come as a surprise: of course social media platforms are monetising every square inch of space they’ve got. However, Messenger sits in a unique spot in the social media landscape. It is the third most popular social networking app (in the United States at least) in its own right, but Messenger is also strapped to the behemoth social media platform that is Facebook itself, which has suffered several hits to its reputation in recent years.

While Facebook use is not drastically dropping despite the platform’s many controversies, many users have clung on to their profiles simply for what Messenger provides: an unhindered messaging experience which, at present, provides the easiest way to stay in touch with the most (or indeed least) important people in your life.

But if Messenger’s new, invasive advertising strategy turns an unhindered experience into one that’s infuriatingly and unrelentingly roadblocked, it could prove to be the final straw drawing barely-there users away from Facebook for good.

For example, let’s say you’re doing something relatively benign – like arranging a lazy Sunday meetup or letting someone know you’ve arrived somewhere; an autoplay video likely won’t be that much of a bother. But let’s think about a more time-consuming or attention-demanding situation, like an argument with your partner, maybe a meaningful heart-to-heart with a friend, or even something just anxiety-inducing and frantic, like trying to sort plans in a time crunch. Before autoplaying video ads, Messenger was the perfect place to conduct such conversations. Now, though, this could become a nightmare.

For those who are merely hanging onto their Facebook accounts to keep in touch with distant relatives or for school friends you reluctantly get a drink with once every two years, this poses a problem. Sure, staying on Facebook to keep using Messenger makes sense when it’s the easiest way to stay in touch, when it’s non-invasive, and simple to use. But introduce video ads for a knock-off Candy Crush after every other message, and those users might find that its worth finding other platforms to host those relationships to avoid the unrelenting irritation.

It’s too soon to tell whether or not these ads will be enough to tip barely-active Facebook users into becoming inactive. But we do know that this roll-out means Facebook is willing to sacrifice a relatively untouched element of their platform that users actually seem to like in its pursuit of revenue. There’s a chance Messenger could introduce ads that aren’t such a painful eye-sore, as Instagram has been able to add autoplaying video relatively seamlessly, and Facebook reported that testing showed that the new ads did not turn users off to using the app. But this move will make many of those who are only using their Facebook account for Messenger reconsider whether it’s worth clinging on.