When posting a new question on any SE, as you type in tags the browser fetches autocomplete suggestions. If you type something that has even one match, you get a very fast drop down list of options. If you type a tag that does not exist, the spinning icon that supposedly indicates the browser is still waiting for a response from the server will not go away until you perform another action (click somewhere, press another key, etc). Using firebug, I confirmed that the ajax request was sent and responded to in under a second but a patient person could easily sit there for minutes waiting for something to happen.
This seems like a UI bug to me, not just because its distracting but because less tech-savvy users may believe the site and/or the tag search is broken.
I suggest that the spinning icon should go away as soon as the client gets a response, even if there are no suggestions to give.
Even better in my opinion, particularly for the sites that will attract a less technology minded crowd than SO, it would be nice if the suggestion box (or somewhere else?) could show an error indicating that no such tag exists and that the user does not (yet) have the ability to create a new tag. I understand that the error is shown after submitting, but for many users, that's too late because they're no longer in the process and so they won't go back to pick from the available tags.
(In case this turns out to be specific to my stack, I'm using Firefox 3.6.10 on Ubuntu 10.04 but I see the same thing in Chrome.)