Bug overview description.
If you attempt to change the namespace, aka the prefix/tag type, of an existing tag in the tag editor, then the error it returns shows html <p> and </p> tags in the output.
What part(s) of the site page(s) are affected?
Wherever you can edit tags and receive a "Post updated" message.
What is the expected behavior?
For the html paragraph tags to not be present.
What actual behavior is given instead?
The html paragraph tags are present when they shouldn't be.
Time of incident (if applicable).
9/1/2019 11:05PM PST.
Can you reproduce the bug every time?
Yes.
https://files.catbox.moe/meeqrx.png
https://files.catbox.moe/3db5cv.png
What steps did you take to replicate this bug?
I simply tried to change the namespace of an existing tag again.
Errors or other messages returned (if any).
<p>Tag type of "[tag]" cannot be changed to "[new namespace]": You can not edit the type of existing tags using type prefixes, or the tag has more than 100 posts. Please review current usage of the tag and if you are confident the tag's type should be changed please use the Tags section of the website to change it.</p>

Did you try the Tags link at the top of the page?

https://e621.net/tag?name=Sander_drevnich&type=&order=count

Search up the tag. To change the tag type hit "edit". you'll have to choose from a drop down menu and hit "save" to lock the tag type as character, artist, general, or copyright.

"Artist:" only works first time if the artist name has never been in the system before.

vex714 said:
-snip-

That tag's actually for a character, and I changed its type to that after I found out.
The main purpose of the screencaps is to demonstrate the bug.

dardranac said: -snip^{^2}-

Oh I see

changing tags via metatags are no longer available to users below privileged rank. For the average member to change a tag type, they have to go to the tags page at the top of the screen and searching the tag.

Big reason this was implemented is that pretence to this change, tracking whom changed a tag type wasn't possible. So if someone vandalized the tags the admins wouldn't be able to know whom did it.

Though this tag takes away the convenience of changing tag types, I am all for it as it greatly reduced the amount of invalid tag changes, as many people wouldn't think to create a new tag for say an artists persona, and instead change the artists artist tag to a character type leaving many posts with having none or unknown_artist in place.

See forum
https://e621.net/forum/show/265727

Versperus said:
-snip-

I'm aware of this. However, yesterday I forgot that you need to do this to change a tag's type and accidentally found the bug.
The issue isn't that you need to use the Tags page.
The fact that there's <p> and </p> tags in the error message is what I'm reporting.