IRC convo, for posterity:

11:58 PM <bearND> tgr: the enwiki TFA (of today) has an overridden description with wikitext inside: https://en.wikipedia.org/w/index.php?title=WestJet_Encore&action=edit.
11:59 PM <bearND> https://usercontent.irccloud-cdn.com/file/CtnV0ntM/Screen%20Shot%202018-09-17%20at%2011.06.08%20AM.png
11:59 PM <bearND> Is there a way wiki text could be useful in a description?
12:00 AM <tgr> not really, but in theory we are converting it to plain text
12:00 AM <bearND> where do we try to do that?
12:02 AM <bearND> Do descriptions show up anywhere else beside in the apps?
12:05 AM <tgr> nah, I'm probably misremembering
12:06 AM <tgr> the logic's at https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/client/includes/Hooks/ShortDescHandler.php#L61
12:06 AM <tgr> you can see the description on the info page
12:07 AM <tgr> also I think the short description template has some kind of hack to show it if you override default CSS with a gadget, but since it uses its own logic for that that one probably does get parsed
12:08 AM <tgr> so anyway, user error, should not contain wikitext
12:09 AM <bearND> ok. Yeah the wikitext shows up even on the info page (https://en.wikipedia.org/wiki/WestJet_Encore?action=info)
12:10 AM <tgr> https://en.wikipedia.org/wiki/Wikipedia:Short_description does not explicitly say it's plain text but does say it should not contain links so you can refer to that if you want to edit it
12:12 AM <bearND> oops, already edited it
12:13 AM <bearND> but i'll keep it in mind if anyone complains
12:19 AM <bearND> tgr: Maybe the description override can display an error when wikitext is used inside?
12:31 AM <tgr> I'm not sure how we van detect wikitext, short of parsing which is expensive. Also the description could be plain text which just happens to be valid wikitext (granted it does not seem likely)

Related: T195551: [BUG] Formatting in local descriptions is breaking use of descriptions (example: San Francisco article)

IMO we could be much more restrictive about what characters/patterns are allowed in local descriptions. No special formatting characters, HTML tags, wikitext markup, etc.

I would think we could catch most of the common mistakes with a few simple regexes to detect things like HTML tags or common wikitext constructs (e.g., [[]], {{}}). It won't foil someone determined to beat the system and enter invalid text, but that should be enough to prevent most good faith mistakes.

For reference, this appears to be the validation logic currently applied to descriptions in Wikibase:

https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/54d283c322062e7e9ed49a6a15738bbe75d4ee10/repo/includes/Validators/TermValidatorFactory.php#L124-L135

/**
 * @return ValueValidator[]
 */
private function getCommonTermValidators() {
	$validators = [];
	$validators[] = new TypeValidator( 'string' );
	$validators[] = new StringLengthValidator( 1, $this->maxLength, 'mb_strlen' );
	// no leading/trailing whitespace, no tab or vertical whitespace, no line breaks.
	$validators[] = new RegexValidator( '/^\s|[\v\t]|\s$/u', true );
	return $validators;
}

In T204592#4612030, @JMinor wrote:

I think originally Gergo's parser function was stripping out some non-allowed characters, but not sure how robust.

It matches Wikidata logic, so it mostly filters out characters which you couldn't type in an input field. { or [ is not invalid in a plaintext description, just unusual.

Making changes in the description that are invisible to the user entering the description is not a good place to be.
Rejecting the edit can be problematic when the description comes from another page (although the only realistic scenario I can think of is it coming from an infobox which generates it from the page title, in which case it is very unlikely to contain anything that happens to be valid wikitext). Also I'm not sure how well it is supported (we do it for more formal content types likes JSON, but do we currently do it for anything in wikitext?) Maybe the enwiki community could set up an abuse filter.

How do the users who make these errors? If they use some kind of tool that shows them the description (like the gadget mentioned on IRC) and that tool makes them believe wikitext works, that should be easy to fix.

@Tgr checking in on this, can the ticket be resolved?

Is this a rare issue or are there hundreds or thousands of such occurrences?

72 (out of 1354718 short descriptions total) per select count(*) from page_props where pp_propname = 'wikibase-shortdesc' and pp_value like '%[[%';. So it does not seem too bad.

We may want to make the description documentation more clearly state that descriptions should be plain text.

Done.

I'll close this as resolved. Please feel free to reopen if you think more (or something else) should be done.

In T204592#5311413, @Tgr wrote:

Another interesting issue that was raised there is the capitalization - the short descriptions guideline recommends using sentence case, while Wikidata doesn't, so users will see inconsistent casing.

Related: T227424: Sentence-casing Wikidata descriptions in the Android app results in wrong case for proper nouns starting with lowercase