Other known demons are spaces (T7305), colons (T7341), and sort-of-but-not-really hyphens (T9480).

I think all this stuff is pretty much case-by-case but fairly straightforward:

• Commas should not be permitted in slugs;
• colons should not be permitted slugs;
• spaces should get normalized/redirected when constructed directly in URIs (this probably hits some other characters/cases too, might just be an Apache config issue);
• hyphen case is probably really a subproject issue although we could add a rule to slam _-_ into - without incurring any real costs I think.

T7341 has some discussion of weird edge cases with periods and projects named #internet.com that are weird but I think probably fine as-is since I believe all the behavior is reasonable and intuitive, it just implies a result that isn't intuitive (namely: we sometimes generate hashtags which are not usable in comments).

At HEAD:

• Hashtags should no longer generate with colons, commas, or other problematic characters.
• /tag/blah/ URIs should be much better about reasonable behavior for unusual inputs.
• Stuff with interword hyphens should generate very slightly nicer tags/slugs.

But note:

• This does not fix anything historically, so you'll have to go clean up the hashtags for existing projects.
• (If you need to swap the primary hashtag for a janky existing project, you may need to rename it and then rename it back to force an update.)