I found one edge case where the new and improved tag editor does not get displayed.
Steps to reproduce (10k users only):
- While looking at a question, click the "edit tags" link next to the existing tags.
- Without clicking either the "Save Tag Edits" or "Cancel" buttons, suddenly realize that you might as well fix some typos at the same time, so click the "edit" link to edit the question.
- The input field for the tags will now lack all of the cool features that it usually has. In fact, not only is the new and improved tag editor not shown, even the old one is missing in action. There appears to be no tag suggestions/auto-completion whatsoever.
This has been successfully reproduced in Chrome (latest version) on Windows 7.