Steps to reproduce:

<gallery mode="packed" class="center">
File:Test.png|a
File:Test.png|b
</gallery>

1. Put a gallery like the above into a page.
2. Save the page, and edit the page with the visual editor
3. Click on the gallery you have added
4. Flick to the "Options" tab and note the contents of the "Classes" field.
5. (Make any change to the page so you can save the page.)
6. Check the diff of the page.

Result:

It adds a redundant, undocumented "classes=" parameter: <gallery mode="packed" class="center" classes="center">

This bug originally reported by @BurritoBazooka