A common and serious flaw I see in student presentations is that the presenter rushes through the first 5-10 slides. This is the worst thing to do in a presentation, these introduction slides to the problem and solution are the most important ones. If the presenter loses the audience in these beginning slides, this wastes the entire hour for the audience. In comparison, losing the audience in the middle of the presentation is less critical, less time is wasted, and the audience can even use this time to think about alternative solutions to the problem (which was presented clearly in the introduction).
You would think this is common sense, but most of the students still commit this mistake. I guess the reason is since the presenter had read and understood the relatively easy introduction part of the paper, he thinks the audience can also understand and follow it with ease. And armed with the powerpoint slides, the presenter can finish the first 10 slides in less than 10 minutes in bullet time. I intercept the presenters with questions if I see this happening, but I am getting really tired of playing the stupid/clueless professor. The worse thing is most of the audience do not follow the presentation, but nobody intercepts. I think in any case it is better to intercept to avoid wasting time for everyone.