Apparently, not all Unicode characters are allowed as tags. For example, the character 😃 (SMILING FACE WITH OPEN MOUTH) can not be added as a tag.
Steps to reproduce:
- Edit a post and add 😃 as a tag.
- Save the post.
- Edit the post again (or go to https://www.bibsonomy.org/tag/😃)::) the tag has disappeared (and the post is not listed on https://www.bibsonomy.org/tag/😃)..)