Say you're in a face-to-face classroom and you discover that you've dropped a week in making up your syllabus. I do this often enough that I should trademark it as "Undine-style teaching,™ now with the magical disappearing week," so this isn't exactly a hypothetical instance. What I do is announce it, thank the person who brought it to my attention, issue an updated syllabus, and move on.
Or you hand out an assignment, and, as you're talking the students through it, some hands shoot up and ask questions about something you hadn't even thought to put on the assignment, because it never occurred to you as an issue. I like to think that my assignments are pretty carefully laid out, but then I hear this: "Do you want paper clips or staples?" or "If I don't use the extra time to do X, can I have it as extra credit points?" I answer the questions, make a mental note to add that information next time, and, yes, move on.
And anyway, it's not always what you did or didn't say. You hand out a quiz, say "this is an open-book quiz," and, two minutes later, five students ask, "Is this an open-book quiz?"
But when you're teaching online, you have to think like a student and try even harder to anticipate these questions from the beginning not just of the class but of the semester. First of all, you're not supposed to change anything once an online class has started.
Second, I've been teaching long enough not to break the cardinal rule: never change a deadline date, because even if it's to their advantage and you make the date later, someone who wrote the paper early will always, always complain about it on the evaluations. I don't like to think what would happen if a date got changed in an online class, where some students like to print out all the course materials on the first day and ignore everything but the Discussion Board thereafter even though they're cautioned not to do this.
Although there's a space for students to ask me questions about the class, and they can email me with questions, it's better if I'm as clear as I can possibly be about what I expect from them. This is always a good policy, of course, but when you can't see them, and they can't see you, it's doubly important. You have to think like a student the whole time. No wonder online teaching takes so much time.