- On https://www.wikidata.org/wiki/Special:NewItem
- Language list dropdown is open
- Type in (say) "es" over default language ("en")
- Spanish gets highlighted
- Mouse cursor happens to be over another entry (say, "de-ch"), which is not highlighted
- Press return key
- Language gets set to "de-ch"
- Language gets set to "es"
Found with Firefox 85 beta (Mac), not tested elsewhere