Things I have experienced first hand, either myself or graduate students:
* A male PI that hires attractive graduate students and gives them easy tasks to land Nature, Science or Cell co-authorships and at the same time tries to land sexual favors from them.
* A female PI that only hires female graduate students and is incredibly aggressive towards male students from other groups, trying to undermine their research.
* A junior faculty member who progressively asked some students to do more and more unreasonable tasks in a particular project so that they eventually refused. Got them removed them from the project clearly in order to get a better position for himself in the publication and to include some of his friends. All this close to submission.
* A group of junior faculty members who plotted to change first authors days before submission, without communicating with the downgraded authors. Needless to say, they upgraded themselves to first authors, without having contributed anything.
* Written threats to misrepresent research results.