This especially when it is picked up by partisan propagandists to push their agenda. It is often not easy to forensically follow the trail of fake news but here is a recent example "caught in the wild".
The Associated Press is a nonprofit and political neutral news agency financed by U.S. newspapers and other media outlets of various political stripes. Its wide range of customers (mostly) prevents it from partisan domestic reporting. It takes on international issues are different. The selection of the news items it reports on is driven by customer interests and thereby slanted in its selection. But the factual reporting on news items is generally straight forward - or supposed to be such. Political decisions are sometimes based on its reports. It is therefore causing concern when it spreads obviously fake news.