Tweeting URL shows wrong domain

I think I answered this question via email, but in case it was a different person with the same issue, here’s my response:

Was this for a post you published while doing development on localhost, or a brand-new post? If it was previously published, you’ll want to go in to edit the post and choose the option to “Clear Tweet History” in the post meta box, so that you won’t continue to use the previous URL.