An issue that has cropped up for me since upgrading to WordPress 3.6, is that when I go to edit another user's post, I can no longer preview unsaved changes. For instance, if I'm notified via Edit Flow that a post has been saved for me to review, I'll go in as usual, and I can preview the post as they've left it, but if I make changes in the post of any kind, I can't preview those changes without saving first. Once I've saved, I can preview the changes without issue. I have superadmin privileges.
This issue is happening on several sites on which I use Edit Flow. It is not evident on sites on which I do *not* use Edit Flow, which is why I have assumed it is specific to Edit Flow.
If I'm editing/previewing posts of which I am the author (even if I am one of multiple authors), the issue does not occur.