A young journalist named Relotius who worked for "Der Spiegel" was found to have his stories a little bit too perfect. He acknowledged that some parts of his stories were "beautified" by him to make them more appealing to the readers.
So, while the core of a story might still be true, nobody really knows what happened and what was added by him. Nobody believes anything of these stories anymore after all.
It totally blew his career up (he was formerly found to be one of the "rising stars" of German journalism and had already got some awards for his work) and the Spiegel also found himself under heavy criticism, although they already have a concept in place, where another editor always fact-checks the other editors work.
So: Enthusiasm in journalists can go really wrong.
I think you are underselling it. Quoting from Der Spiegel[1], "he included individuals in his stories who he had never met or spoken to, telling their stories or quoting them. (...) He also made up dialogue and quotes."
[1] http://m.spiegel.de/international/the-relotius-case-answers-...
[1] https://digiday.com/media/inside-spiegels-70-person-fact-che...
The problem with that is that the person with the byline isn't the only one that matters. Editorial oversight is equally important. You need the guy who's enthusiastically digging into a story and the guy who keeps his enthusiasm in check. I don't think those can be the same person.