I find it very easy to mistakenly make new tags by mistyping an existing tag's name. I've also had the issue where I somehow changed the very first tag in the list, when adding a tag a new tag to a question. For example here football was changed to efootball when I added the equipment tag to the question, and here basketball became pbasketball when adding postseason. For some reason the first letter of the new tag I added to the list, also got applied to the first tag that was already in the list.
It'd be nice to have a pop up letting us know we're creating a new tag, just in case it wasn't the intended goal, specially since it's a functionality of SE.