I was editing https://www.mediawiki.org/wiki/Wikimedia_Engineering/2015-16_Q3_Goals using VisualEditor. I made a bunch of changes over the course of five minutes, and clicked save. I'd made so many changes over the five minutes that I wanted to preview my edit. To my surprise, the preview showed me removing a bunch of content that was not on the page when I started editing it. I was quite concerned, but since I know how to fix this kind of thing, I just saved my edit anyway.
After saving my edit, I checked the diff of my edit, and it was 100% correct and didn't show me removing any content. But, I did see an intermediate edit, between when I clicked edit and when I showed the preview, which added content that looked like the content the preview showed me removing.
I want to stress that, in the end, the edit was saved 100% correctly as I expected it to be with no removal of content. That's great! But, the preview was wrong.
Below are some steps that I think will reproduce the bug, but it's hard to say because I'm not certainly exactly what happened.
- Load a page
- Open VisualEditor on that page
- Using a different browser logged into a different account, load the same page, make some changes, and save the page.
- Go back to the first tab. Make some changes. View the wikitext diff.
At this point, the wikitext diff will (likely) show you removing the content that you added using the other browser.