1. Go to https://en.wikipedia.org/w/index.php?title=Template:X1&oldid=840783873
2. Test the link. It should give you a 404 error due to the escaping.
3. Open the revision in the 2017 wikitext editor.
4. Add a space, and then preview the revision.
5. Test the link. It should work, due to an incorrect escaping implementation.