/help/privileges/create-tags reads as follows:
Simply enter a new tag with your question and it will be created. When creating your new tag, bear in mind that tags ...
- must be shorter than 25 characters
- must use the character set
a-z
0-9
+
#
-
.
This is false - tags must be at most 25 characters in length (i.e. 25-character tags are permitted).
(This is also a doc bug at least on ja.SO, where it reads 「25 文字未満」, which means "shorter than 25 characters".)