Here is a story AP published that turned out to be fiction. It originated with Jon Ralston and was quickly picked up by AP and then other major media outlets. No corroboration ever materialized for the story, despite there being 3000 cameras in the room, and those that streamed live told a different story. The only media outlet to retract the story was NPR, and PBS fired Ralston.
Note also that this is the story that the DNC leadership instructed its members to pass around "without attribution", i.e. covertly smear Sanders with it. It's also the email that Assange has singled out as the most damming.
And also remember that much of the brouhaha over the leaked DNC emails was over collusion with the media.
In short, if you aren't yet skeptical of mainstream media this year, you need to start paying closer attention.