In T258201 we enabled the lead paragraph transform to apply to non-infobox elements. However this broken the debug logging that was in place for https://logstash.wikimedia.org/goto/04520c8fceeb291e778f6cfcf4daf6dc
Usually we see < 50 errors an hour, but since this change we've been seeing > 5000.
The debug logging/transformation logic should be adapted to handle such elements.
The error occurs when the lead thumbnail is inside an unexpected container, eg. a div,table, or gallery.
Logic in https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/includes/Transforms/MoveLeadParagraphTransform.php#L190 should only apply to elements with the infobox class.
QA Results - Prod