One thing we can do is increase the parameter-finding to avoid {{{#, because that's never the start of a valid parameter name.

I think that catches the errors reported here (this was with this template wasn't it?) but there are constructs like this that don't work:

{{{party{{#if:{{{party_election||}}}|_election||}}|}}}

It doesn't find party, but I think that's acceptable isn't it? There are always going to be ways to make things that won't be guessable. (Also, why does this have two pipes after party_election?).

Change 459886 had a related patch set uploaded (by Samwilson; owner: Samwilson):
[mediawiki/extensions/TemplateData@master] Prevent leading hash from starting param names when guessing

https://gerrit.wikimedia.org/r/459886

Change 459886 merged by jenkins-bot:
[mediawiki/extensions/TemplateData@master] Prevent leading hash from starting param names when guessing

https://gerrit.wikimedia.org/r/459886

This will be testable on testwikis when train resumes next week.