Writing an academic paper with multiple authors can be problematic at times (for examples see this article and comments on the article), but when do you even make people a co-author?
There are problems across scientific fields with co-authors being added who did not contribute significantly to papers, for example heads of labs or departments, or prestigious individuals (so- called “honorary authors”). Some laboratories even have a policy of adding everyone in the lab who even passed by a manuscript, in order to bulk out resumes.
Individuals who warrant co-authorship, but who are left off the publication (so called “ghost authors”) are also an issue. One of the most common examples of this is when an ambitious faculty member leaves off a student who conducted majority of the work (or who possibly even came up with the idea) because they want first (or possibly sole) authorship for the paper so that they can further their academic career. In the biomedical field ghost authors are often pharmaceutical industry representatives who may rewrite sections of manuscripts to show their product in the best light, but exclude themselves from authorship and thus obfuscating conflicts of interest. Such conflicted ghost authors are not unique to the biomedical field though, and industry, military or governmental ghost authors have frequently been known to substantially rewrite (and change the conclusions of) marine environmental science papers, especially when they deal with controversial topics.