I have put some fancy unicode emoiji characters in my OSM username, and you can add the #OpenStreetMap wiki (which is #mediawiki) to the list of software which barfs on it.
@rory Do you have a link or any more details so I could reproduce the issue? I'd be interested in fixing the issue upstream in #MediaWiki itself (if that's where it is).
A Mapstodon instance for the OpenStreetMap Community (English language)!
This site is under construction, and may change.
Our francophone friends have a Mastodon instance: fr.osm.social