One of the built-in example queries for the Wikidata Query Service is "French people with an article on the English Wikipedia but not on the French one". However when you try to run this query, it gives you an error:

QName 'schema:about' uses an undefined prefix

https://query.wikidata.org/#PREFIX%20wd%3A%20%3Chttp%3A%2F%2Fwww.wikidata.org%2Fentity%2F%3E%0APREFIX%20wdt%3A%20%3Chttp%3A%2F%2Fwww.wikidata.org%2Fprop%2Fdirect%2F%3E%0APREFIX%20wikibase%3A%20%3Chttp%3A%2F%2Fwikiba.se%2Fontology%23%3E%0APREFIX%20p%3A%20%3Chttp%3A%2F%2Fwww.wikidata.org%2Fprop%2F%3E%0APREFIX%20v%3A%20%3Chttp%3A%2F%2Fwww.wikidata.org%2Fprop%2Fstatement%2F%3E%0APREFIX%20q%3A%20%3Chttp%3A%2F%2Fwww.wikidata.org%2Fprop%2Fqualifier%2F%3E%0APREFIX%20rdfs%3A%20%3Chttp%3A%2F%2Fwww.w3.org%2F2000%2F01%2Frdf-schema%23%3E%0A%0ASELECT%20%3Fitem%20%3FitemLabel%20%28COUNT%28distinct%20%3Fsitelink%29%20as%20%3Fcount%29%20WHERE%20{%0A%20%20%20%3Fitem%20wdt%3AP31%20wd%3AQ5%20.%0A%20%20%20%3Fitem%20wdt%3AP27%20wd%3AQ142%20.%0A%20%20%20%3Fsitelink%20schema%3Aabout%20%3Fitem%20.%0A%20%20%20FILTER%20EXISTS%20{%20%3Fwen%20schema%3Aabout%20%3Fitem%20.%20%3Fwen%20schema%3AinLanguage%20%22en%22%20}%0A%20%20%20FILTER%20NOT%20EXISTS%20{%20%3Fwfr%20schema%3Aabout%20%3Fitem%20.%20%3Fwfr%20schema%3AinLanguage%20%22fr%22%20}%0A%20%20SERVICE%20wikibase%3Alabel%20{%0A%20%20%20%20bd%3AserviceParam%20wikibase%3Alanguage%20%22fr%2Cen%22%20.%0A%20%20}%0A%20}%20GROUP%20BY%20%3Fitem%20%3FitemLabel%20ORDER%20BY%20DESC%28%3Fcount%29