Steps to reproduce:
- Use Firefox (probably)
- Use the New Topic Tool to post a ==New section==
- See something weird (rarely)
After posting, the new section is displayed at what should be the end of the page, but parts of the DiscussionTools interface doesn't get removed, and the page contents get duplicated underneath the new section. In this screenshot, the ==Lead image== section is the new one, and it's at the end of the page ...even though it doesn't look like it on screen.
The resulting wikitext is good (diff for the edit showin in this screenshot), and reloading the page solves the display problem.