- Be logged out of VE, or otherwise in single-tab mode
- Visit https://en.wikipedia.org/wiki/User:Whatamidoing_(WMF)/sandbox?veaction=edit
- Note the URL changing to https://en.wikipedia.org/wiki/User:Whatamidoing_(WMF)/sandbox?veaction=edit&action=edit
- Reload the page
- Find you're now on the non-VE version instead
The likely vector of this happening would be someone else sharing a link, since if you're in single-tab mode the links we generate will be in the action=edit form already.