The tag picker currently ignores hyphens when you are typing a tag, and will prevent non-diamond moderators from creating a tag with hyphens when one without exists (or vice versa).
I've encountered a lot of people creating new tags for things with dots in their name when there is already an existing tag with a different dot status.
js. Unfortunately tagging for these libraries is amazingly inconsistent, which can make it hard for users to find the right dotted or undotted tag. This is sometAngularJSimes because the libraries themselves are inconsistent. backbone.js and angularjs, for example. I've found myself fixing angular.js once a week. We've also recently had sailsjs and sails.js coexist accidentally, as another real, live documented example.
The problem is amplified by component and plugin names that alternatively use periods or hyphens in their name.
This can also happen in languages that use periods to separate names, or where the period is part of the name. I'm looking at you,
.NET. I see someone doing things in asp-net-mvc occasionally when they should be doing things in asp.net-mvc. Hell, today we had aspnet-mvc. 100% preventable.
Adding periods to the list of ignored characters would greatly reduce the number of users creating a new tag when an existing one should be used.