Reproduce:
- Open any Module editing page e.g. https://www.wikidata.org/w/index.php?title=Module:Property_documentation/sandbox&action=edit
- Paste the following to debugging console and press enter: mw.wikibase.getLabelWithLang("L3302-S1")
First reported in https://www.wikidata.org/wiki/Wikidata:Project_chat#Property_talk:P5972_has_weird_error_message