I added the property "Commons category" (P373) to a lot of items, see for example https://www.wikidata.org/wiki/Q1008 . It contains a string with the name of the category on Commons.
On the Dutch Wikipedia I added some parser functions to find problems with the data, see https://nl.wikipedia.org/w/index.php?title=Sjabloon%3ACommonscat&diff=37414305&oldid=36545480
I noticed a large number of pages with "funny" characters in https://nl.wikipedia.org/wiki/Categorie:Wikipedia:Commonscat_met_lokaal_andere_link_dan_op_Wikidata . This category is for items which have a local variable set and it's different than the one set on Wikidata. Some pages which ended up in there shouldn't be there. Take for example https://nl.wikipedia.org/wiki/Ivoorkust , it links to the exact same category on the Dutch Wikipedia as it's object on Wikidata.
My assumption is that somewhere encoding got mangled up.
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