- Go to https://en.wikipedia.org/w/index.php?title=Template:X1&oldid=840783873
- Test the link. It should give you a 404 error due to the escaping.
- Open the revision in the 2017 wikitext editor.
- Add a space, and then preview the revision.
- Test the link. It should work, due to an incorrect escaping implementation.
[[toollabs:templatecount/index.php?lang=en&namespace=10&name=High-use|This link works in preview but not when saved]]
Gah. The Parsoid link looks correct -- but in fact question marks should be escaped in interwiki links. That's actually an existing bug: T95473: Interwiki links whose titles contain question marks are not properly escaped in HTML output.