In the comments to https://gerrit.wikimedia.org/r/483490 (for T213468) @Arlolra found a possible corner case where Parsoid and PHP might diverge in edit-section numbering (the numeric parameter found in the "edit this section" link): we process <includeonly> and <noinclude> in a subpipeline. This shouldn't matter for most pages, since we section numbering is suppressed inside included templates anyway, but if you were directly viewing a page in the Template namespace which included these elements -- and that page included section headers which are otherwise balanced and Parsoid-approved -- it is possible Parsoid and PHP might diverge. That would be bad, because you'd get the wrong section to edit and (in the worse case) your edited text could get reinserted in the wrong section of the page.
Filing this task to investigate as to whether there is an actual issue here, and fix it if so.