When an example listed on https://www.mediawiki.org/wiki/Wikibase/Indexing/SPARQL_Query_Examples has an extra prefix, it is not used when selecting this example through the "Query Examples" selector of https://query.wikidata.org. Thus, the query ends with a java exception "QName '<some label>' uses an undefined prefix".

You can try with the following examples:

• Aliases of properties which are used more than once (should have the prefix for skos:)
• French people with an article on the English Wikipedia but not on the French one (should have the prefix for schema:)