I just tried to post a question with a Latin alpabet tags, and have been notified that tag a is not allowed. Why it is not allowed?
Despite of allowing any other single letter tag.
Seems this affects all of SE sites.
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There are a few very common words that are not allowed as tags -
a and a few others (this is configurable, so the list can change, which is why I am not listing all of them).
I believe this was done so edits would get rejected when a user simply writes a sentence in the tag input instead of actual tags.