Among the presentations in a major conference, I attended one where the author cited a paper (of theirs) under review in another major conference. Not only cited it, but showed some of its results.
I have the impression that it should not be possible, since there's the potential risk of influencing reviewers who could also be in the room during the presentation. Or is it only "good practice" not to do so?
The only problem of presenting results that are not published is if someone else steals the results and writes their own paper about them. (Though this probably more likely only happens with ideas that are shared too prematurely.)
It is generally considered a good thing to promote one’s own work, and one way of doing this is by giving presentations at other universities.
Even if this work is under review at a different conference, I don’t think it is problematic. When presenting a paper at a conference, you are not necessarily obliged to talk about precisely the contents of the paper. You are advertising the paper, and more generally, your own work, so that people will read it and cite it. If you have bigger and better results, then these will help with your promotion of your own work.
Of course it would be weird, though probably not wrong, to talk entirely about a different paper when presenting at a conference.