I recently tried to edit Template:Infobox country on en.WP. I made a minor fix to correct a typo, and I received an error message when I attempted to save: "Syntax error in JSON (help)". I subsequently noticed multiple problems:
1. I tried to save again, and the editor let me save the template.
2. I edited the template again to make another small fix, got the same error message, and was unable to save despite four or five attempts. Note that I was NOT trying to modify any JSON, about which I know almost nothing because I have found it to be poorly documented.
3. I clicked on the "help" link, and it took me to a page that did not have a section describing how to respond to this "Syntax error in JSON" message. Please create such a section, knowledgeable people.
4. I was finally able to deduce/guess that the problem was in the template's documentation, not in the template code, even though I was not editing the documentation. I happened upon the link to JSONlint and was able to fix the documentation, which then (apparently) allowed me to save the template.
5. I was able to re-edit the documentation to introduce invalid JSON, with no error message or complaint from MediaWiki: https://en.wikipedia.org/w/index.php?title=Template%3AInfobox_country%2Fdoc&type=revision&diff=914345844&oldid=914345261
Please apply the JSON error-checking code only to the page that contains the JSON formatting information, NOT to the template. At a minimum, please apply the JSON error-checking code to any page containing JSON code so that JSON-deficient/clueless editors such as myself do not get stuck when trying to make valid wikicode edits to template pages. Whoever introduces the invalid JSON code to the documentation should be prevented from saving, not the subsequent template editor.
Rant end. Thanks for reading.