Initial transcluded <p> elements on some pages are fooling extractLeadIntroduction into outputting gibberish lead intros. This issue was initially discovered on dewiki but might affect others.
intro: "<p><span style=\"display:none\"><span class=\"new\">p2</span></span></p><span>\n</span>"
This of course affects summary output for the affected pages as well, since the summary is derived from the lead intro.
The 02 Feb 2018 page summary switchover was delayed at the last minute due to the eleventh-hour discovery of this issue.
There's currently a workaround strictly for page summaries on master but this needs to be fixed in extractLeadIntroduction.