Add an event to a SOGo calendar and add an emoji (I have tested smiley face, birthday cake, and basketball) to the title. The event will be saved to SOGo and is even visible in the web interface, but iOS and Thunderbird clients will no longer be able to completely sync the calendar. You can verify this by removing the caldav account on iOS and re-adding it. In my case, none of the events on the calendar were visible until I removed the emoji event in the SOGo web interface. In Thunderbird (38.6.0), a "CalDAV: Fatal Error doing multiget for Personal Calendar" will appear in the error log until the offending event is removed.