Today, I manually removed the misspelled tag [tag:registraton] from two separate questions. While this was an easy thing with only two questions having the tag, either **making the misspelling a synonym** or deleting it and putting it on the blacklist will prevent it from happening again. And it is an easy misspelling to make and a hard one to notice when gazing at the word. 

While I changed the tags, I also noticed that there is a [tag:signup-signon] tag. While we could make a case that if you nitpick it, [tag:signup-signon] can be used for questions which refer to both, and [tag:registration] should be reserved to questions which are only about signon, it seems that the users are not nitpicking. They frequently use [tag:signup-signon] in combination with either [tag:registration] or [tag:login]. When it is used alone, such as in http://ux.stackexchange.com/questions/6203/password-field-in-cleartext, one of the other two would obviously be also appropriate, in this case [tag:registration]. 

So my second suggestion is to **burninate [tag:signup-signon]**, as its usage shows that it is redundant. The separate words, [tag:signup] and [tag:signon], could be created as new tags and be made synonyms of [tag:registration] and [tag:login] respectively. Simply making the double tag a synonym does not seem possible, as it is usually applied to questions which refer to only one of the two concepts.