I've seen this issue occur with at least 2 of our instructors, and it's probably happened with a few more instructors, they just haven't alerted us (our office).
The issue: after creating an assignment page, and then going in to edit it (add content) and then save it, the instructor receives a "Page Not Found" page. The content isn't there, the assignment page isn't there. But, if the instructor leaves this area and comes back in sometime later, eventually the assignment page is there and the content is there.
Is this potentially related to the cache? Or something else? I've seen this on both a Mac and a PC.