Seen in Logstash since 1.32.0.wmf.13, does not appear to be present in wmf.12 and earlier so likely a recent regression.
[{exception_id}] {exception_url} ErrorException from line 83 of /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/Services/WikiTextContentCleaner.php: PHP Notice: Undefined index: 3
[{exception_id}] {exception_url} ErrorException from line 81 of /srv/mediawiki/php-1.32.0-wmf.13/extensions/FileImporter/src/Services/WikiTextContentCleaner.php: PHP Notice: Undefined index: 2
- /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/Services/WikiTextContentCleaner.php:83
- /srv/mediawiki/php-1.32.0-wmf.13/extensions/FileImporter/src/Services/WikiTextContentCleaner.php:81
message: PHP Notice: Undefined index: 3 exception_url: /wiki/Special:ImportFile http_method: POST server: commons.wikimedia.org exception.trace: #0 /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/Services/WikiTextContentCleaner.php(83): MWExceptionHandler::handleError(integer, string, string, integer, array, array) #1 /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/Services/WikiTextContentCleaner.php(41): FileImporter\Services\WikiTextContentCleaner->cleanTemplates(string) #2 /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/Remote/MediaWiki/ApiDetailRetriever.php(275): FileImporter\Services\WikiTextContentCleaner->cleanWikiText(string) #3 /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/Services/SourceSite.php(68): FileImporter\Remote\MediaWiki\ApiDetailRetriever->getImportDetails(FileImporter\Data\SourceUrl) #4 /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/SpecialImportFile.php(267): FileImporter\Services\SourceSite->retrieveImportDetails(FileImporter\Data\SourceUrl) #5 /srv/mediawiki/php-1.32.0-wmf.14/extensions/FileImporter/src/SpecialImportFile.php(172): FileImporter\SpecialImportFile->getImportPlan() #6 /srv/mediawiki/php-1.32.0-wmf.14/includes/specialpage/SpecialPage.php(566): FileImporter\SpecialImportFile->execute(NULL) #7 /srv/mediawiki/php-1.32.0-wmf.14/includes/specialpage/SpecialPageFactory.php(569): SpecialPage->run(NULL) #8 /srv/mediawiki/php-1.32.0-wmf.14/includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext) #9 /srv/mediawiki/php-1.32.0-wmf.14/includes/MediaWiki.php(867): MediaWiki->performRequest() #10 /srv/mediawiki/php-1.32.0-wmf.14/includes/MediaWiki.php(524): MediaWiki->main() #11 /srv/mediawiki/php-1.32.0-wmf.14/index.php(42): MediaWiki->run() #12 /srv/mediawiki/w/index.php(3): include(string) #13 {main}