- Go to /wiki/Some_page?oldid=123 where 123 is the oldid of the most recent revision (e.g. https://en.wikipedia.org/w/index.php?title=User:Roan_Kattouw_(WMF)/Sandbox&oldid=731550113 ; go to the history and click the topmost timestamp)
- Click the edit tab to open VE
- Make changes
- Click edit again
- Be surprised that you're in oldid mode and don't see the changes you just made
If you do this with any other oldid (i.e. the ID of an actually-old revision), you will now be editing the most recent revision of the page. However, because the oldid is the most recent ID, VE gets confused and goes into oldid mode. We should never be in oldid mode on the second edit.