From fishing through error logs in PHI184. When row 1 has less cells than row 2, and row 2 has a |--| section (indicating that the cell above should be a table header), we try to convert an invalid cell into a table header. PHP obliges and creates this cell, but it doesn't have 'content', which produces this error:
ERROR 8: Undefined index: content at [/core/lib/libphutil/src/markup/engine/remarkup/blockrule/PhutilRemarkupBlockRule.php:160]