Rescinding offers of admissions I got some admissions offers from Ph.D. programs in the United States. In the near past, I worked as a research assistant for a professor. The advisor was completely unethical and abusive. All his past students without exception say bad things about him: he writes negative letters of recommendation to his students even after agreeing to write strong letters and in some cases, he did send negative letters un-invited to schools that accepted some of his M.Sc. students. I am very happy now that I left him. I mentioned in my CV that I worked as a Research Assistant but I did not back it up with reference letters of course. In case this professor finds out that I received offers and tried to contact these schools, is there a possibility that they might decide to rescind their offers?
All his past students without exception say bad things about him: he writes negative letters of recommendation to his students even after agreeing to write strong letters and in some cases, he did send negative letters un-invited to schools that accepted some of his M.Sc. students.


Short of a serious, substantiated allegation of research misconduct, we would take a letter like this to be weird and immediately disregard it (with a few choice comments about the professor involved --- in a small field I suspect we might already know that this professor has some issues). 

If an offer had already been sent out, it wouldn't be rescindable based on this. I'd wager to say that this is probably the same case across all public North American institutions.