1. On enwiki Special:RecentChangesLinked page enter the page title that will fetch some results with the default page settings - e.g. Apertium.
2. Change the page tittle to de:Apertium - the suggested title will be displayed in the drop-down:

The results will not be displayed - a user might think that it's a correct result (which is not true).

3. Reload the page - the warning explains that the pages titles with invalid interwiki prefix cannot be used here. The prefix de is not invalid, so the warning is misleading.

4. Remove the prefix and observe that the results are displayed, but the warning does not go away:

Note: the steps can be repeated with entering an invalid page title - e.g. <<MediaWiki:Tag-foo-description>>, and then changing it to a valid title.