Steps to reproduce:
- Go to Beta Commons
- Search for a string (scroll to "containing..." in the autosuggest box)
- execute the search
- Get the search results page which displays a yellow warning box with the text: "A warning has occurred while searching: Mixing entity and article namespaces in search is currently not supported. Only entity namespaces will be searched."
@EBernhardson did an initial look at the bug (and hopefully he'll provide those insights here ^_^) but it also needs a fix.
Note: Beta Commons may still have Wikibase Federation turned on which could account for differences from production and test commons