I've found that I can create (from the settings page) a tag containing a '/' which then can't be removed. There's no displayed error when this happens; the settings page just reloads but the tag is still listed.
I can also create a tag containing a ',', but when I try to remove it I get the "Fatal Error (500)" page.
Note that this basically contains #1246, but that one was closed (erroneously, I think) because the importer no longer creates tags containing commas. However, you can still create those tags from the settings page and the problem with removing them remains.
This has been reported again in https://pagure.io/pagure/issue/2854
Metadata Update from @pingou: - Issue tagged with: bug, easyfix
Commit 772157fd fixes this issue