Scope Freakout, otherwise known as the part of the project where every requirement that can be documented has been documented. I don't think I've been a part of a large project that hasn't had it.
As a PM, I take a lot of responsibility for that moment. Mind you, dear reader, I have tried valiantly to keep all my business users in the loop to avoid it. And in my case, it turns out we really hadn't missed much.
But let me share my very common mistake. We say a project needs regular status meetings, which we have. Often, we split the detail meetings into business and technical camps (and with good reason). And I do that with the best of intentions, which are to minimize the time impact on everyone. So I cut meetings down to "need to know" groups.
Don't do that so much.
I'm not saying every meeting should be all-hands, dragging everyone through the minutiae of every small feature. What I will do in the future is to schedule regular, all-hands line-by-line reviews of each requirements document at least three times before they go to development.
Your mileage may vary. Mine never does, though I seem to constantly have to re-learn this lesson.