Steps to Reproduce:
- Enable “Show previews without reloading the page” in Preferences / Editing.
- Open a page for editing with useskin=cologneblue, e.g. https://en.wikipedia.org/w/index.php?title=User:Tacsipacsi&action=edit&useskin=cologneblue
- Click Show preview.
An error message appears with API error unknown_useskin.
The preview appears.
(Note that although this bug is most easily reproducible using the live preview feature and the Cologne Blue skin, I don’t think any of them are buggy—live preview is simply a front-end to the API, and Cologne Blue is a skin hidden from preferences.)