What wrong
If the user use the char "#" in the reference name (minimal) as the first char, so:
In the HTML output is the <a> link with two chars "#". That is invalid (See 1, 2).
**Example of the wrong situation
The right way
the # character will remain allowed for users, but in the internal source will have the char "#" another transcription.
Looking at the changes in https://ja.wikipedia.org/w/index.php?title=%EF%BC%BBAlexandros%EF%BC%BD&diff=prev&oldid=89972724 and then https://ja.wikipedia.org/w/index.php?title=%EF%BC%BBAlexandros%EF%BC%BD&oldid=89972724 (please provide direct links, step by step)
In the HTML output is the <a> link with two chars "#".
I cannot find any ## in the HTML of the page.
in the output page it doesn't work the reference tooltip.
That's the local gadget https://ja.wikipedia.org/wiki/MediaWiki:Gadget-ReferenceTooltips.js . User scripts, gadgets, templates, custom CSS are local on-wiki content. Local content is managed independently on each wiki, by each wiki community themselves. This needs fixing on the local wiki. Hence I am closing this task here - thanks for your understanding!
@Aklapper: I mind "with 2 chars #", but none "with double char #". Wikimedia take the reference name and then add the custom prefix with first char "#". Then in <a> tag is the output <a href="#someonthing#someonthing">. And that is none valid HTML. The right is <a href="#someonthing%23someonthing">.
Hi, please don't report two different things in one bug report...
Bad value #cite_note-#1-32 for attribute href on element a: Illegal character in fragment: # is not allowed.
See also T20431
In the HTML output is the <a> link with two chars "#". That is invalid.
This is incorrect, # in the fragment part of an URL is *not* supposed to be percent-encoded. It must only be encoded if it occurs in the path or the query. See https://url.spec.whatwg.org/#fragment-percent-encode-set