I've seen a beautiful bug on my forum (ABM ) :
1- you make a topic with a tag [restricted] in an area restricted to a group of users
2- you connect as an user from this group and click on the tag [restricted] -> you see the topic
3- you connect as an user lambda (with no rights), you can see the tag [restricted] even if it's not in any viewable topic. I you click on it, no topic will be seen.
4- if you connect again as a user from the restricted group, the tag will have disapeared, and if you click on it on the topic, no results will be seen