Editing kerning classes from the font-info view does not invoke the illegal-character-check. This allows to specify invalid kerning classes which are silently ignored on export.
The invalid class name even shows up in the edit view, where changing the class name does display an error dialog.
While emojis are clearly not allowed, I assumed a forward slash is legal and since Glyphs did not stop me, it took me some time to figure out why my kerning pairs did not work.