"Um, will this be on the midterm, and by the way, when will that be?"
Every time this happens (and it actually happens a lot), you can physically observe the lecturer's train of thought derailing and exploding into a mangled pile of molten wreckage. If the professor manages to answer all of the disruptive student's concerns, they then have the difficult task of re-engaging the other 25-100 students whose concentration has since gone elsewhere.
It's incredibly rude to ask a professor whether or not the material that they are covering will be on an exam. What you're essentially asking is, "do I need to be paying attention to you right now, or can we all just zone out while you talk at us?" That thought process represents a very immature mindset that still thinks you're paying for "an education" and not paying to actually learn.
That said, I don't think that this is something that most people will understand given the rise in tuition costs and the (flawed) notion that students are paying for a 4-year "educational resort and spa" experience. As someone who got an undergrad degree then worked for a few years before going back to grad school part time, it's amazing how much I learned to appreciate the classroom environment. IMO students must not realize how infrequently after graduation they'll have the opportunity to learn new things from someone more knowledgable than themselves.