I've edited a few tags on Meta, and it appears that the tag dropdown isn't always working. I then end up creating new tags, when I might have chosen an old one.
I think you may have a caching problem with the jQuery files that run a lot of the AJAX on the sites. You can test this by trying to click the Flag link on a post and see if you get the red menu pop up. If not, you should try and clear you browser cache and initiate a hard refresh on your browser. That should clear it up.