Here I wanted to add a word within a link, which didn't work out fine. So I switched to source code editing. This resulted in doubling all parts of the article outside the paragraph I chose to edit first. Plus, it removed the article from my watchlist.
Topic on 2017 wikitext editor/Feedback
Thanks - this looks like a bug switching from VE after using a section edit link.
See https://phabricator.wikimedia.org/T153276 (fixed in master)
This is now deployed to the wmf.6 production branch, which means it should be everywhere except Wikipedias right now, and will be fixed on Wikipedias too within about an hour's time. Thank you for spotting this!